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Modules API
Javadoc
You may want to look at the
Javadoc,
particularly that for the
ModuleInstall
class. Other classes in the package are generally not useful to module
authors.
Contents
Modules API
What are Modules?
Modules permit NetBeans to be extended
dynamically. All of the NetBeans APIs are designed to be used for purposes
of implementing modules. Modules may range in complexity from a single
Java class, properly packaged, to do something elementary such as add
a menu item to the Edit menu to display the contents of the clipboard;
to a full-scale integration of a major external application, such as a
Java profiling suite.
All modules are distributed and installed as JAR files. The basic
format should be rather familiar; classes constituting the module are
archived in the JAR, and special entries in the
manifest file are recognized.
Related Standards
To the greatest extent possible, NetBeans has designed the module
system to reuse standard technologies when they are sufficient, and to
follow the style of others when not.
The basic idea for the format of modules is taken from the
Java Extension Mechanism.
The basic ideas behind the
Package Versioning Specification
is used to handle dependencies both between
modules and of modules to the system.
All modules have some set of basic properties that indicate which
types of features they provide, which Java classes implement these
features, and what special option settings should be used when
installing these features. Some of this information is
listed in the
manifest file
using the customary format, and NetBeans-specific attributes. The
Java Activation Framework,
as well as JDK-internal features such as support for executable JAR
files, was used as a model for how to specify the useful contents of a
JAR in a data-driven fashion: many modules will need no special
installation code other than attributes in the manifest, and an XML layer
giving additional more specific deployment information.
How to Create a New Module
For basic cases, you should need to do very little to create a module
besides writing the basic source code.
Build a JAR
All module implementation classes must reside in a JAR file. If you
want to split up a large application into several pieces, perhaps
so as to make independent upgrades possible, you should do so by
creating multiple modules and relating them using versioning.
Writing the manifest
A module is recognized as such by NetBeans, by virtue of its having
a special magic tag in the global section of the manifest:
OpenIDE-Module. Its value should be an arbitrary
identifier (with a format similar to that of a Java package name)
identifying the module for purposes of upgrades and
dependencies. You are encouraged (but not required) to use as this
tag the name of a Java package containing the principal classes for
the module; this ensures that there will not be conflicts between
modules produced by different developers or institutions, provided
that you follow the JavaSoft recommendations on naming packages
according to an Internet domain name you control. See the section
on versioning for details on what this tag
is used for.
There are a few other global tags which are optional but encouraged:
OpenIDE-Module-Name
- Gives a human-presentable display name for the module. The name
may be localized by adding additional tags for each locale,
e.g.
OpenIDE-Module-Name_fr.
OpenIDE-Module-Short-Description
- A short description of what the module does (about one sentence, like
a tool tip). May be localized as for the display name.
OpenIDE-Module-Long-Description
- A longer description of what the module does (roughly
paragraph-length). May be localized as for the display name.
OpenIDE-Module-Display-Category
- A phrase giving a category for the module. Modules with the same category may
be visually grouped together in various parts of the UI.
May be localized as for the display name.
OpenIDE-Module-Install
- The name of a custom module class.
OpenIDE-Module-Layer
- The name of an installation layer.
- Various versioning- and dependency-specific tags
- Please see the section on versioning
for details on these.
OpenIDE-Module-Module-Dependency-Message and
OpenIDE-Module-Package-Dependency-Message
- Localizable pleasant messages to display to a user in case
a module or package dependency fails. In some cases it may be
quite normal for a dependency to fail and it is desirable to
provide a specific and helpful message to the user explaining
where to get the required dependency or why the module depending
on it is not needed by this user. May be localized as for the
display name. Since 1.26
OpenIDE-Module-Requires-Message
- As above, localizable pleasant message to display when a required
token is not found. Since 2.3
The presentation-oriented tags (display name and category, short and
long description) may be in HTML if you prefer; this should be done in
the standard Swing fashion, by beginning the text with the magic
string <html>.
Since 1.24:
A more flexible way to provide human-readable information is to declare
the attribute OpenIDE-Module-Localizing-Bundle. Its value should
be the resource path to the (base locale of) a bundle file providing localizable
attributes for the module (display name, category, short and long description).
The bundle file may have localized variants as usual, and the keys should be the
same as the desired manifest attribute names, e.g. OpenIDE-Module-Name.
If it is necessary to localize an attribute from a section (currently only filesystem
sections with their Display-Name attribute), you may again place these
in the bundle, where the key will be prefixed by the section name and a slash, e.g.
org/foo/MyFileSystem.class/Display-Name.
All other tags are bound to a particular module
section. Naturally you may use other standard manifest attributes.
In summary, here is an example manifest file that could be included
with the JAR tool's -m option:
Manifest-Version: 1.0
OpenIDE-Module: com.modulemakers.clip_disp/2
OpenIDE-Module-Specification-Version: 2.0.1
OpenIDE-Module-Implementation-Version: 2.0-beta-rewrite
OpenIDE-Module-Build-Version: 2003-12-31 00:23 [email protected]
OpenIDE-Module-Name: Clipboard Displayer
OpenIDE-Module-Name_cs: Prohlizec schranky
OpenIDE-Module-Install: com/modulemakers/clip_disp/Installer.class
OpenIDE-Module-Layer: com/modulemakers/clip_disp/Layer.xml
X-Comment-1: I am a comment (just a deliberately meaningless
X-Comment-2: header) - "Sealed" is a standard manifest attribute.
Sealed: true
Name: com/modulemakers/clip_disp/DisplayClipboardAction.class
OpenIDE-Module-Class: Action
Module authors are strongly encouraged to use the
module development support in the NetBeans IDE
which among other advantages, provides interactive parsing of
manifests that can help the beginning API developer immediately see
how NetBeans will parse his manifest, including possible parse errors.
If you have troubles with a manifest, check to make sure you have
the suggested extra blank line at the end. Some JDKs may have trouble
parsing it otherwise.
Using a custom module class
With the OpenIDE-Module-Install attribute, you may specify
a custom class which will handle any complex aspects of the module's
installation (or uninstallation). This class is only necessary to
write if the standard module sections and
layers do not
cover everything you need to do. Even if you do write such a class,
standard sections and layers may still be used for any part of the module's
integration which is conventional - the main class need
only handle the exceptional parts.
To use a main install class, just extend the
ModuleInstall
base class. Your class must be able to be
instantiated as a JavaBean.
There are several methods which you may override, and may do anything
which is required to make the module cleanly enter and exit NetBeans.
For example:
package com.modulemakers.clip_disp;
import org.openide.modules.ModuleInstall;
import org.openide.filesystems.FileUtil;
import java.net.*;
public class ModuleHandler extends ModuleInstall {
public void installed() {
// This module has been installed for the first time! Notify authors.
HttpURLConnection conn = (HttpURLConnection)
(new URL ("http://www.modulemakers.com/clip_disp/installed.cgi").openConnection ());
conn.getResponseCode ();
conn.disconnect ();
// Handle setup within this session too:
restored ();
}
// Nothing special required here.
// public void restored() {
// }
// Do not need to do anything special on uninstall.
// Tools action will be removed automatically.
// public void uninstalled() {
// }
public boolean closing() {
// Ask the user to save any open, modified clipboard contents.
// If the user selects "Cancel" on one of these dialogs, don't exit yet!
return DisplayClipboardAction.askAboutExiting ();
}
}
The installed handler may do more or less what it
wishes - it will be called in a running NetBeans instance. The same applies to
uninstalled and closing. However,
restored is more delicate in that it may be called
during earlier phases of NetBeans initialization. Therefore, it should
not use presentation-oriented features of NetBeans, such as the
Explorer or Window System APIs. However, the other APIs are
acceptable to use in restored, including
DialogDisplayer.
The
ModuleInstall.updated(...)
method will be called just once when
a module is updated to a new version - when the new version is loaded
into NetBeans for the first time (in a new session), the method is called
and the previous version number is accessible. Neither installed
nor uninstalled will be (automatically) called in this case.
It is a module author's responsibility to make sure that new versions of a
module are capable of "cleaning up" obsoleted installations created by older
versions, as far back as a user is likely to be directly upgrading.
In some cases, a module relies on some external resource to be present
in order for it to be used. For example, an external application may need
to be installed in order for it to do anything. Or it may require a valid
license key. In such cases, the
ModuleInstall.validate()
method may be overridden to check for this external resource and throw
a (preferably politely localized) IllegalStateException
if something is wrong. Throwing this exception will cancel the module installation
(or restoration) cleanly. Note that such an installer should not expect
anything related to the module to already be loaded when this method is
called, as it may be done when deciding whether to even begin loading.
"Installation-clean" modules
Since NetBeans can better manage modules when their resources are
declared to exist, rather than procedurally installed, it is desirable
to use declarative APIs whenever possible. Consider the following
definition of an "installation-clean module":
- Either the module has no specified
ModuleInstall; or
- It does have a
ModuleInstall but this installer:
- Does nothing of consequence during initialization of the
ModuleInstall object (static or instance fields with
initializers or initializer blocks or the initialize
method).
- Does not override
installed nor updated
(these will then just call restored).
- May override
uninstalled but only to undo the effects
of restored or other changes made by the module
while running. May override closing and/or
close but only to undo the effects of changes made by the
module while running.
- May override
restored and/or validate.
Additionally it is recommended not to use restored to
add an object to the running NetBeans instance where use of the module layer would
suffice, nor to modify persistent state in restored or
validate that might affect a future call to the same
method. When possible it is best not to have a
ModuleInstall at all.
Furthermore, the following practices should be avoided wherever
possible in order to simplify installation of the module from Auto
Update and via "ad-hoc" addition of the module JAR by a user (note
that mentioning options here does not imply that they are
officially supported by the APIs or even unofficially possible at any
particular time):
- Including any libraries in the lib/ext/ directory, as
these cannot be added to or reloaded while NetBeans is running.
- Using the docs/ directory as a place to store module
JavaHelp rather than inside the JAR. (
@HelpSetRegistration
makes this obsolete anyway.)
- Using
Class-Path from within the module JAR or its
extensions to point to resources which are not in or below the
directory containing the referring JAR (i.e. inclusion of
../ or absolute paths).
- Including any files in the NetBeans installation other than the module
JAR in the modules/ directory and any extensions declared
directly or indirectly via
Class-Path beneath the
modules/ directory (for example in
modules/ext/) and any locale variants of the above JARs;
except insofar as the presence of such files does not in and of itself
change the functionality of NetBeans and is not required for the module
to operate (for example mounts of documentation ZIPs declared in the
layer); even then this is discouraged if there is an alternative as it
complicates ad-hoc module installation though Auto Update may be fine.
Using an installation layer
You may specify the global tag
OpenIDE-Module-Layer in addition to or instead of a
module main class; as a rule, use a layer for a particular task in
preference to a module installer, if you can. The tag value should
point to an XML file inside the module which is in the format
understood by XMLFileSystem.
Its contents specify some files that the module will add to the
system filesystem
controlling much of NetBeans' configuration. This filesystem is composed of many read-only XML layer filesystems
as well as a writable layer corresponding to the config subdirectory of the user directory.
For many APIs, there is no need to write a layer explicitly; layer-generating
"registration" annotations do everything you need.
The XML format is relatively simple. There is an
online DTD
for it, but descriptively: the document element is
<filesystem> and corresponds to the root folder;
subfolders are represented with the <folder>
element; and files within folders with the <file>
element. Files and folders must have names, with the name
attribute. Files need not specify contents, in which case they will be
zero-length; or they may specify contents loaded from some other
resource (url attribute, treated as relative to the base
URL of the document itself); or for small textual contents, the
contents may be included inline inside the element, typically using
the CDATA syntax, though this usage is deprecated.
Attributes may be specified on folders
or files as empty <attr> elements; the name must be
supplied, and the value in one of several formats: primitive types,
strings, URLs, arbitrary serialized objects in hexadecimal, or
computed on the fly by calling the default constructor of some class,
or a static method (the method may be passed the file object in
question and/or the attribute name, if you wish).
The primary advantage of using layers is that they are declarative
rather than procedural, so you can install many kinds of extensions to
NetBeans' behavior without any Java code, only XML. A related
advantage is that unlike ModuleInstall, there is no need
for multiple kinds of logic for module installation, restoration,
uninstallation, and upgrading; the "files" added by the layer are not
stored to disk and are loaded by NetBeans in every session from the
XML, so when the module is uninstalled or upgraded its associated
files are correctly removed or changed, respectively, without any
extra work. (If the user customizes the files, however, their
customizations are stored to disk, and thus form a permanent
"patch" against the baseline configuration provided by the module.)
Not all parts of NetBeans' configuration can be customized using files on
the system filesystem and thus by layers, but many things can.
Some common things that layers tend to be used for:
- Install
actions
into NetBeans' global menus, toolbars, or keymap.
- Create templates in the folder Templates/.
- Other things such as bookmarks, Component Palette beans, Welcome
panel buttons, and so on - according to the
structure
of the system filesystem.
- Most generally, to install arbitrary
services
into the system.
By default files listed in XML layers are not ordered in any
particular way, just as files found on disk would not have a
particular order. For purposes of some kinds of installation, it is
desirable to order data objects in data folders in a particular way -
for example, menus are
built from their menu items
in the order the instance-bearing data objects occur in the data
folder. The call
DataFolder.setOrder(DataObject[])
suffices to arbitrarily change folder order, but usually it is
desirable to create the proper order declaratively in the XML. For
this reason, DataFolder understands special attributes
(set on the files in the folder) which order the objects in the folder.
Specifically: if there are data objects A and B in the folder,
represented by
primary files
afile and bfile,
and afile has an attribute named position whose numeric value is less
than the corresponding attribute on bfile, then the folder will attempt to
place A before B in its ordering. For example, the XML:
<folder name="SomeFolder">
<file name="first.txt" url="first.txt">
<attr name="position" intvalue="100"/>
</file>
<file name="second.txt" url="second.txt">
<attr name="position" intvalue="200"/>
</file>
</folder>
will cause NetBeans to try to keep first.txt before
second.txt. Things to remember:
- If the user reorders the folder manually or
DataFolder.setOrder(DataObject[]) is called, the explicit
order overrides the existing positions. Subsequently added
constraints might have an effect, however.
- Only the names of primary files from data objects have
any bearing on the ordering.
There is also an older, now-deprecated system based on relative ordering attributes.
More details are available in FileUtil.getOrder.
The resource path pointing to the layer is automatically localized
by NetBeans every time the module is installed or restored; if there
are locale-specific variants, they are merged with the main
layer. More-specific variants can simply add files to the set
installed by the module; however they take precedence over the
less-specific variants, and thus can replace files, or even remove
(suppress) them, using *_hidden masks as used by
MultiFileSystem.
In fact, if module A depends on module B, then A's layer(s) will take
precedence over B's layers, and it may replace or remove files
installed by B. In the same way, any module may replace or remove
files installed by the core.
Display-oriented file attributes
NetBeans recognizes several special file attributes, typically set in layers, which can control UI.
For example, the labels of menus created from folders beneath Menu/
are derived from the "localized display name" of the layer folder, when defined.
See FileSystem.getDecorator()
(formerly getStatus())for details.
Reverting user's modifications
When a user makes any changes in the installation layer - usually through
the user interface by changing the default values for some application options
or any other customizations - it may be desirable to revert these changes back
to their defaults as defined in XML layers. There changes are stored in user's
working dir.
Since version 7.2 the FileObject supports a new attribute
"removeWritable" returning an instance of
Callable which removes the local writable version of the
given FileObject thus reverting the folder or file to
its initial state as defined in XML layers. Please note that is *not*
possible to reset FileObject's attributes.
Using versioning
Module versioning provides a way for modules to specify
which module they are (i.e. that one JAR is an upgrade from
another); which version they are; whether they have introduced
incompatible API changes since a previous version; and
(importantly) whether they depend on other modules or system
features, and if so how. While very simple modules may not require
special versioning support, this system should be used for any
module published on a general release schedule, or which it is
expected other modules may want to make use of.
Modules can do three things with versioning:
- Specify what they are. This is done by the special
OpenIDE-Module tag, whose value should be a unique
(programmatic) identifier for the module, as mentioned above. Not
be confused with the display name, which is free-form and may be
changed at will, this code name should not be changed
arbitrarily - pick a name and stick with it throughout module
releases.
- Specify which version they are. In line with the Java
Versioning Specification, modules can indicate two pieces of
version information about themselves using the
OpenIDE-Module-Specification-Version and the
OpenIDE-Module-Implementation-Version tags.
Modules are also permitted to use OpenIDE-Module-Build-Version to
give information about when or by whom they were physically built, in case there
is more specialized semantics given to the implementation version.
- Specify which features they depend on. Again, this is done
using the Versioning Specification - modules can request general or
specific versions of other modules
(
OpenIDE-Module-Module-Dependencies), Java packages
(OpenIDE-Module-Package-Dependencies), or Java itself
(OpenIDE-Module-Java-Dependencies).
The tags
OpenIDE-Module-Provides and OpenIDE-Module-Requires
can also be used to specify dependencies between modules without naming
the exact module to depend on. A module may provide one or more
tokens. These are strings of conventional meaning, in the format of a Java
package or class name - perhaps taken to be the name of a class which will
be supplied to the lookup system, though there could be other meanings. A module
may also require one or more tokens. A module which requires some tokens
may only be enabled by the system if for each such token, there is at least one other
module which provides that token and is already enabled.
Since version 7.1 there is also support for OpenIDE-Module-Needs
which is a weaker version of requires as it does not impose any restriction on
the ordering of module. The OpenIDE-Module-Needs is useful for
modules that define some API and require an implementation of it. Just
specify the need
for an implementation and make other modules depend
on your module and OpenIDE-Module-Provides the implementation token.
Moreover there is also OpenIDE-Module-Recommends which is even
weaker version as it creates a conditional dependency - e.g. enables the module
providing the token only if it is available, however if it is not, no dependency
is broken.
At the heart of all these tags are the conventions used in the
Package Versioning Specification.
While this documentation should be referred
to for the full details and justification of this system, the basic
idea is that features (modules) have two significant version identifiers: a
specification version, in Dewey-decimal format
(e.g. 1.2.1), and an implementation version,
which is some free-form text. Specification versions may be
incremented to indicate compatible API extensions, in which case
the check to make sure that the feature requested is available may
be done using a (lexicographical) compare on the Dewey-decimal
numbers (e.g. 1.0 < 1.0.1 <
1.1). Implementation versions may be requested
according to an exact match only. Thus, specification versions are
generally used when you depend on some general and specified
behavior of another feature; implementation versions are used to
tie builds of different features together closely, as when you
maintain and release all the features yourself, and wish to
distribute them in a well-tested unit.
(You may also specify a build version in your manifest,
typically giving a date. This is used only for diagnostic purposes and
is otherwise ignored by the module infrastructure.)
So, modules may specify either or both kinds of versions in
their manifest, and correspondingly request such versions from
other modules. (You may have at most one dependency between any single pair of modules, however.)
The Versioning Specification provides similar
numbering for (some, but not all) Java packages, as well as the
core Java Platform APIs and Virtual Machine Specification.
NetBeans also has such versions which may be used to request a
general level of Open API support (using specification versions) or
a particular NetBeans build (using implementation versions).
Since the Versioning Specification does not address incompatible
changes (those which would break existing code), but these are in
practice occasionally necessary (however painful),
modules may provide a separate integral number
giving the incompatible release version. You may only
request a particular release version, not subsequent ones, and you
must specify the release version if there is one
when giving any module dependency. It only differs
from a complete change of the feature in that the module installer
tool may prompt the user to make a "major upgrade" if a new release
version of a module is available.
Since 2.3 a module dependency may also use a range
of release versions. The syntax is that in place of a single major release
version, you give a minimum then maximum version separated by dashes:
OpenIDE-Module-Module-Dependencies: base.module/1-2 > 1.0
where the base module might either have the minimum major release
version (and the given specification version or later, if that is also
requested), or other major release versions up to the maximum (in
which case the specification version is irrelevant). You may not ask
for an implementation version with this syntax.
To use all of these tags once you understand them is not too
hard. First of all, feature names: a module is named according to
the OpenIDE-Module tag, which should look like a Java
package name, but may be followed by a slash and release number
(e.g. com.mycom.mymodule/3);
Java packages are just named by the package name; the Java Platform
APIs are named by Java; and the Java Virtual Machine
by VM.
Now, a module may list its own specification and/or
implementation versions using the tags above. To specify a
dependency on other features, it may use some or all of the four
dependency tags. The value of each will be a comma-separated list
of dependencies of that general type, each of which can be of the
form:
- FEATURENAME
- Just requests that the feature be present, not any particular
version. Useful for modules and Java packages (usually, Java
standard extensions) that need to be installed, but that is all.
This style is also used for requiring tokens, which cannot have versions.
- FEATURENAME
> SPECVERSION
- Requests that the feature be present, that it state a
specification version, and that this version be greater than or
equal to the requested version. Note that you may not request
an exact specification version. This is the preferred form for module
dependencies.
Remember, if requesting a dependency on a module which has a
release number, you must give that release number as part of
the feature name, whether or not you also ask for a specification
version.
- FEATURENAME
= IMPLVERSION
- Requests that the feature be present, that it state an
implementation version, and that this version exactly match the
requested one. Note that you may not request a comparison on implementation
versions, as they are generally non-numeric and not comparable.
In summary, here is an (unlikely) example that specifies and
requests all sorts of versions:
OpenIDE-Module: com.mycom.mymodule/1
OpenIDE-Module-Specification-Version: 1.0.1
OpenIDE-Module-Implementation-Version: 1.0-release-g
OpenIDE-Module-Module-Dependencies:
com.mycom.mysistermodule/1 = 1.0-release-g,
com.othercom.anothermodule > 2.1,
org.netbeans.modules.applet/1 > 1.0
OpenIDE-Module-Package-Dependencies: javax.television > 0.9
OpenIDE-Module-Java-Dependencies: Java = 1.2.1b4, VM > 1.0
OpenIDE-Module-Provides: javax.television.TunerProvider, javax.television.RemoteControl
OpenIDE-Module-Requires: org.netbeans.javahelp.api.Help
There is further special treatment for handling of package
dependencies for several reasons:
- In practice, the Java Versioning Specification is not all that
widely followed, and many needed extensions will not list this
information in their manifest.
- Class loaders only define packages when the first class from that
package is actually loaded. This makes it more difficult to verify
whether a package is really there or not.
- The standard manifest attribute
Class-Path must be
recognized and used; typically this attribute is used to actually add
extensions to the module classloader, while the Modules API attributes
are used to ensure that specific versions are available.
You may specify a sample class name in square brackets immediately
after the package name (or indeed instead of it). Giving a class name
together with a package name requests that NetBeans first try to load
that class (it must be able to, so choose a class in the extension you
know will be there); then the versioning information is checked. (If
the sample class is simply in the desired package, you may omit the
package qualification as a shortcut.) This
addresses the second problem. Giving just a class name in square
brackets with no preceding package name indicates that NetBeans should
only ensure that the named class is loadable, bypassing the Versioning
specification mechanisms. This addresses the first problem.
Here then is a sample manifest which depends on an extension called
again javax.television, where it is known that a class
javax.television.TunerProvider is part of the package.
The extension is stored in modules/ext/television.jar
relative to the cluster directory:
OpenIDE-Module: com.mycom.mymodule/1
OpenIDE-Module-Package-Dependencies: javax.television[TunerProvider] > 0.9
Class-Path: ext/television.jar
Where can you get the version information to depend upon?
- For installed modules, this information is listed in the
module's node in the Modules area under Session Settings.
This includes information about provided tokens.
- For packages, use the Java
Package
class, or examine the manifest of the JAR you depend upon.
- For the Java platform, use the system properties
java.specification.version and
java.version; for the VM,
java.vm.specification.version and
java.vm.version.
- For NetBeans, this information is printed to the console during
start-up, and also appears in
var/log/messages.log.
Important! If your module has compile-time references
to classes in another module, you must specify a
direct dependency on that module using OpenIDE-Module-Module-Dependencies.
This ensures that the modules will be
installed in the correct order; otherwise a
NoClassDefFoundError or similar problem could result.
Since 2.19 modules which provide Java-level APIs can
specify that only certain packages in the module form part of the
public API and should be accessible to other modules depending on
that module. The tag OpenIDE-Module-Public-Packages
is optional; if not present, all packages are considered
fair game. If present, it gives a list of package names to export
to other modules, for example:
OpenIDE-Module-Public-Packages: org.netbeans.api.foo.*
indicates that only classes (and resources) in the package
org.netbeans.api.foo should be available to client
modules. Attempts to use classes from other packages will fail,
for example with a NoClassDefFoundError.
More than one package can be given; separate them with
commas or spaces. A package name may end in .** rather
than .* to indicate that any subpackages are also exported,
rather than just the package itself. The special value -
(a single dash) indicates that no packages are to be exported.
(The module might still provide a non-Java-level API, for example by
defining and handling a custom XML DTD.)
"Private" classes and resources
may still be accessed using reflection from the system classloader,
just not from a module classloader.
Additionally, sometimes a module with an API in public packages wishes to
provide access only to certain "friend" modules. This is
possible if the module declares public packages together with a list
of code base names of "friend" modules that can access them:
OpenIDE-Module-Friends: org.foo, org.boo
Only enumerated modules can access the public packages;
access from others is forbidden.
Additionally, sometimes a module wishes to get unrestricted access
to non-public packages of an API module. This is discouraged, but
possible if such module declares a direct dependency on the
API module using an implementation version dependency - this kind of
dependency indicates that the client module is prepared to track every
idiosyncrasy of the API module, and knows how to safely use undocumented
classes, as if both modules formed part of the same code base.
Confused by all these restrictions on classloading? See the
Class Path
document which gives a fuller, more informal explanation.
Writing an OS dependent module
Since 4.44 a module can request to be enabled only on certain class
of an operating system
(see issue 46833).
This is done by requesting a special token to be present and the system makes sure that
these tokens are enabled only when NetBeans run on requested class of OS.
The tokens supported in version 4.44 are:
OpenIDE-Module-Requires: org.openide.modules.os.Windows
OpenIDE-Module-Requires: org.openide.modules.os.Unix
OpenIDE-Module-Requires: org.openide.modules.os.MacOSX
In version 6.3 additional tokens have been added:
OpenIDE-Module-Requires: org.openide.modules.os.OS2
OpenIDE-Module-Requires: org.openide.modules.os.PlainUnix
Version 7.3 further added:
OpenIDE-Module-Requires: org.openide.modules.os.Linux
OpenIDE-Module-Requires: org.openide.modules.os.Solaris
Please note, that Mac OS X is in fact also Unix, so requesting Unix also
enables your module on Mac OS X. If you want all unix types but Mac OS X
you want to request PlainUnix token.
So if one wants a module to be automatically enabled on Mac OS X and silently
disabled on other platforms the best way is to create an eager module which
requests the MacOSX token and the module system takes care of
enabling it only on Mac OS X (as is the case of the NetBeans
applemenu module).
Localization and branding support
Existing Internationalization, Localization, and Branding
documentation describes how IDE and platform is localized and general principles are
valid for all modules.
Standard Module Sections
Formerly some items in NetBeans were installed via JAR manifest sections using
the OpenIDE-Module-Class attribute. All such registrations are now
deprecated, generally replaced with layer-based registrations.
Deployment of Modules
This section describes how a module may be deployed to a user's NetBeans instance.
Enablement
The standard module system permits modules to be in several states, controlled
by external configuration:
- Unknown. Either the module is not present at all, or the JAR is present
but its config file is missing or masked.
- Disabled (regular). The module is present and known but not loaded.
- Enabled (regular). The module is present and loaded.
- Autoload. The module will be enabled if and only if some regular enabled
module requires it as a dependency (perhaps indirectly).
- Eager. The module will be enabled if and only if all of its dependencies
can be satisfied without enabling any additional regular modules.
Regular modules are used when their functionality is somehow visible just by
virtue of being enabled - typically because they make layer registrations.
Since this functionality might or might not be wanted it is important for the
user or deployer to retain control over the enablement status.
Autoload modules are used for libraries. If there is no "client" of the
library, there is no purpose in loading its JAR at all. Only when some
regular module requires it (directly, via tokens, etc.) will it be loaded.
Eager modules are often used for "bridges" between otherwise independent
pieces of functionality, represented as regular modules. If both of those
modules are enabled, the eager bridge will be as well, integrating them
using some optional service. They may also be used as add-ons to regular
modules distributed via other channels; or as platform-specific modules
that will be enabled only according to operating
system tokens.
Note that these enablement states are not intrinsic to the module, which
is why they are not specified in the JAR; they are part of its deployment.
Another module system might not make any use of such distinctions. In
particular, modules deployed via JNLP or OSGi are simply enabled by virtue
of being included in the deployment set, or at the mercy of the container.
Related Files
Sometimes a module may need to bundle other files besides the
module JAR alongside it. No concrete mechanism for doing so is
specified in the Modules API. However, if there is such a mechanism,
then it is possible for the module to find these other files.
InstalledFileLocator
permits such packaging mechanisms to let modules find their associated
files, as of 3.21.
In the current implementation, modules may be
bundled with other files using
NBM files
as provided by Auto Update, and the NetBeans build process premerges
modules in the selected module config into one large installation
directory, accessible via several system properties. Correspondingly,
the default locator finds files in this structure. The installation
structure might however change in the future, so
InstalledFileLocator is the only safe way to find such
bundled resources.
Deprecation
Sometimes you may have an API module which you wish to deprecate in its
entirely - it is only available for purposes of backwards compatibility.
Normally such modules are autoloads. To ensure that remaining clients of the
module are properly warned of its status, you may specify the
manifest attribute OpenIDE-Module-Deprecated with the value true.
Generally you should also supply a message using the localized manifest attribute
OpenIDE-Module-Deprecation-Message.
This
message may be displayed somehow if a non-deprecated module depending on your
deprecated module is enabled.
Refactoring
Sometimes as the developer of an API module, you may wish to make
major changes in how your APIs are laid out physically in modules.
Most commonly, you determine that it would be desirable to split one
big API module into several smaller pieces. However you wish to retain
compatibility for existing client modules, so that if they used your
old monolithic module, they will now automatically get access to the
newer, smaller pieces.
This is straightforward: create an XML file according to the DTD
-//NetBeans//DTD Module Automatic Dependencies 1.0//EN
and
install it
in the system filesystem under ModuleAutoDeps/
(system/ModuleAutoDeps/*.xml). Note that it will probably
not work to do this using an XML layer since the information
must be available before any module is even loaded.
As of 3.33
Module install order
It may be possible for multiple modules to be installed at once, or
for a module to specify a dependency which is not satisfied. How
does NetBeans handle such cases?
First of all, when NetBeans starts up, all previously-installed
modules are restored, calling the restored methods on
their install classes if present. (Sections are installed before
this method is called.) If multiple modules are being restored
(which is normally the case), NetBeans installs them in an
arbitrary order. However, if some of them specify
dependencies on other modules, the order may be adjusted so as to
make sure that the one specifying the dependency is installed after
the one it depends on.
Now, when NetBeans is running, modules may be installed into it,
possibly a cluster of them at once. In such a case, NetBeans first
checks to make sure that all the prospective modules satisfy their
dependencies, either on system or Java features; already-installed
modules; or other modules in the prospective cluster. If any
modules fail their dependencies (or had syntactical errors in their
manifest files, etc.), they are removed from the list and the user
is given an explanation of what is missing. The remainder are then
ordered according to dependencies as above, and then installed in
that order.
Caution: it is forbidden to install modules which have
cyclic dependencies on one another, as NetBeans would be unable to
determine which order to install them in! In fact, it will signal
an error and not install such modules. Generally such a situation
means that you should "factor out" the common required
functionality into a new module which the original ones will then
specify legal dependencies on.
List of all modules
Though a module should not generally need to know or care what other
modules are installed - anything it needs should be a dependency, and
anything which can use it, it should not know about - in a few cases it
is desirable to obtain the list of all modules in the system. This can
be easily accomplished as follows:
Collection<ModuleInfo> modules = Lookup.getDefault().lookupAll(ModuleInfo.class);
The resulting ModuleInfo objects give basic information
about known modules (both enabled and disabled), such as their names
and versioning information, and current enablement status. This API is
strictly read-only and informational.
A limited ability to modify the installation status of modules is
provided by the APIs as of version 1.31. In the folder
Modules/ on the system filesystem, there will be XML
files corresponding to all known modules. In each case the name of the
XML file is derived from the code name base of the module with
. replaced by -, and followed by
.xml; for example,
org-netbeans-modules-properties.xml. The contents of the
XML file are given by a fixed
DTD.
For purposes of the API, only parts of these files are defined. The
root element must be <module> and its
name attribute must be the code name base of the module.
Inside the root element are various <param>
elements, each with a name and some textual contents. The APIs define
only one parameter, enabled, which if present must be
either true or false.
Autoload (and eager) modules do not use the
enabled parameter, since their enablement status can never be
specified directly. Rather, they are marked with a parameter named
autoload (resp. eager) with the value
true.
The APIs do not currently permit arbitrary changes to these files.
Specifically, addition or deletion of these files, or modification of
other files that may be in the same folder, is not permitted.
Modification of these files is permitted in only one way: if a module
XML file already contains the parameter enabled, you may
rewrite the file to be identical except with the opposite value of
this parameter. The module system will then make a best effort to
enable or disable the module accordingly (subject to dependencies and
other constraints).
Doing this is not recommended except in unusual circumstances, and
is intended primarily for session support to be able to enable and
disable modules via layer. If your module just needs to prevent itself
from being turned on under some circumstances (for example if it is
missing a valid license key), simply use
ModuleInstall.validate.
When rewriting the module XML be sure to use
FileSystem.runAtomic(FileSystem.AtomicAction)
to wrap the reading of the old XML and the writing of the new, to
prevent file changes from being fired halfway through.
Like any file in the system filesystem, these configuration files are loaded
from the config/ subdirectory of all registered clusters including
the user directory plus XML layers. Normally a module's config file is simply
bundled with its installation and that is what
is used; user-initiated changes are stored in the user directory. But clusters
can also override configuration from other clusters, for example. And the
presence of a Modules/*.xml_hidden mask file will suppress
knowledge of the existence of a module from the system (not just
disable it).
Security
In the current API there is no particular provision for security in
modules: all modules are assumed to be trusted, and have access to
all NetBeans' features (including the abilities of the Java VM
running as an application) if they require them. So, users should
not install arbitrary modules from potentially dangerous
sources. Given the density of callbacks and the fine-grained object
model of the APIs, providing a thread-based security model for API
code is not feasible.
Code which creates new classloaders and loads other code indirectly
should, however, be aware that the created classloaders are generally
subject to regular security restrictions, unless code is loaded from
within the NetBeans installation, or certain forms of NbClassLoader
are used. When in doubt, explicitly define the protection domains for
classloaders you create.
NetBeans currently loads each module in its own classloader,
which means illegal dependencies between modules will result in errors.
The Update Center supports certificate-based signing of
downloaded modules, so that the user can be sure that the contents
have not been tampered with or accidentally corrupted since their
creation.
JNI
It is possible to run modules making use of JNI native implementations inside
NetBeans. You may place the native libraries (DLL or shared-object) beneath
a modules/lib directory (i.e. a subdirectory lib/
beneath the directory where the module JAR resides). If your native library file
names for different architectures or operating systems clash, you may create
subdirectories under modules/lib for each supported platform and
nested subdirectories for each supported operating system. The directory names
must match System.getProperty("os.arch") and
System.getProperty("os.name").toLowerCase(Locale.ENGLISH) respectively.
The System.loadLibrary call originating from the module code
will try to locate the library file in the following order of directories:
- modules/lib/
- modules/lib/<arch>/
- modules/lib/<arch>/<os>/
so you may place e.g. 64-bit Linux version of a foo library in a file
modules/lib/amd64/linux/libfoo.so.
The module may distinguish additional library variants itself or even override
the platform selection logic by naming the library file appropriately
and calling System.loadLibrary with such a name. Use of JNI is of course not
recommended and should be restricted to cases where it is unavoidable.
Example of usage with a fully specified library name: if on Linux a module located in /home/app/cluster/modules/something.jar calls
System.loadLibrary("stuff-linux-i386-gnome"), the library should be in
/home/app/cluster/modules/lib/libstuff-linux-i386-gnome.so. On 64bit Windows, if
C:\app\cluster\modules\something.jar calls System.loadLibrary("stuff"), the library
should be in C:\app\cluster\modules\lib\amd64\stuff.dll. (Remember that Java has platform-specific
prefixes and suffixes it adds to plain library names before trying to find it on disk.)
If a native library refers to other native libraries, they
are likely to be found using the normal search path for the platform.
This may mean that if you have several libraries used in a module, you
must either put all but the directly referenced one in the system's
global library search path, or merge them all together.
Warning: since the JVM cannot load the same native
library twice even in different classloaders, a module making use of
this feature cannot be enabled more than once in a single VM session.
JARs loaded from the classpath (application classloader) are never
reloaded, so it may be possible to include the native-dependent
classes in such a JAR and make use of it from a module JAR.
NetBeans currently includes a JNA wrapper module which can also be used
for native access. As of this writing, however, this module exports only a
friend API, so it is not available for use from arbitrary modules.
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