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Builder to launch your Java/HTML based application.
Essential support for those who write native methods communicating directly with JavaScript.
Mix your Java and JavaScript code seamlessly - perform calls from Java
to JavaScript and back with as much freedom as JavaScript gives you
and as much type safety you can get from Java. Execute your code
in a headless testing environment or in a
JavaFX WebView.
When done, deploy to real browsers.
Simple Meaning of World
The whole support is build around @JavaScriptBody
annotation. Use it to create parametrised JavaScript snippet easily
accessible from Java:
{@code @}{@link net.java.html.js.JavaScriptBody}(args = {"x", "y"}, body = "return x + y;")
private static native int meaning(int x, int y);
The above defines method meaning which sums two JavaScript
objects together (being invoked inside of a JavaScript interpreter).
The meaning method now becomes a properly typed Java
surface to your JavaScript code which can be directly
called from the rest of your Java code:
public static void main(String... args) {
assert 42 == meaning(40, 2) : "Meaning of World should be 42!";
}
Real code tip: real classes using this technique are
available online:
JsMethods and
Bodies.
Editing hint: one can see the list of arguments of the
meaning is now duplicated - it is once specified in Java,
and once inside of the {@link net.java.html.js.JavaScriptBody}
array of args. This is necessary to keep the names of
arguments accessible during runtime. However don't despair - there
is a code completion for the value of args attribute!
Just type the Java signature first and then press Ctrl+Space and the
right parameter names will be inserted for you.
Including JavaScript Libraries
Large amount of JavaScript code is easier to be delivered in whole
files rather than {@link net.java.html.js.JavaScriptBody small code snippets} -
that is also possible thanks to {@link net.java.html.js.JavaScriptResource}
annotation. Imagine file mul.js with following content:
function mul(x, y) { return x * y; }
Place the file next to your class and reference it with
{@link net.java.html.js.JavaScriptResource the annotation}:
{@code @}{@link net.java.html.js.JavaScriptResource}("mul.js") class Mul {
{@code @}{@link net.java.html.js.JavaScriptBody}(args = { "x", "y" }, body = "return mul(x, y);")
public static native int multiply(int x, int y);
public static void main(String... args) {
assert 42 == multiply(6, 7) : "Meaning of World should be 42!";
}
}
All the Java methods annotated {@link net.java.html.js.JavaScriptBody}
can now reference everything that is in the mul.js file -
e.g. the body of the multiply method can reference the
function mul and use it.
Real code tip:
this
is the way
the knockout.js library
is included in its ko4j library.
Callback to Java
Often JavaScript code needs to call back into the Java classes.
For example when a button in a browser is pressed and your code would
like to invoke a runnable to handle such situation:
{@code @}{@link net.java.html.js.JavaScriptBody}(args = {"id", "r"}, {@link net.java.html.js.JavaScriptBody#javacall() javacall} = true, body = "\n" +
" document.getElementById(id).onclick = function() {\n" +
" r.{@code @}java.lang.Runnable::run()();\n" +
" };\n" +
" ")
public static native void onClick(String id, Runnable r);
As can be seen, there is a special syntax (starting with @) to
properly identify the right Java method to call on a Java object passed
into the JavaScript interpreter. The syntax starts with a fully
qualified name of the class, followed by :: and name of the
method including signature of its parameters. In case of runnable,
this is just () as the method has no parameters, but the
signature can be more complicated. For example in case of following method
static int compare(int i1, String s1, int i2, String s2)
it would be (ILjava/lang/String;ILjava/lang/String;) (btw. the
return type is not included in the signature). The actual parameters
then follows. The JavaScript call to such compare method would then
look like:
{@code @}the.pkg.Clazz::compare(ILjava/lang/String;ILjava/lang/String;)(1, 'One', 2, 'Two');
This syntax gives enough flexibility, helps to properly select one
of overloaded methods and follows the tradition of previous attempts to
provide JavaScript to Java calling conventions.
Please note that to turn the special Java callback syntax on, one
needs to set the {@link net.java.html.js.JavaScriptBody#javacall()}
attribute to true. The callback syntax consists of
following parts:
[instance.]@classname::methodname(signature)(arguments)
- instance - must be present when calling an
instance method and must be absent when calling a
static method
- classname - fully qualified name of the class in
which the method is declared
- signature - internal JVM method signature
(as specified at
JNI type Signatures)
without the trailing signature of the method return type
- arguments - the actual values to pass to the called Java method
Here is the JNI type signatures table
one can use to convert
Java parameters to JVM's internal letter based
representation:
Type Signature
Java Type
Z
boolean
B
byte
C
char
S
short
I
int
J
long
F
float
D
double
L fully-qualified-class ;
fully-qualified-class
[ type
type[]
Editing hint: The callback syntax may seem complicated at
first, however there is an associated annotation processor
that checks the syntax and verifies the referenced class and
method with the requested signature exist. If it does not, the
compilation fails offering correct alternatives.
Thus don't despair seeing the syntax, make sure you get the
fully qualified name of the callback class right.
You'll get warning and help
if there is a typo in the specified signature then -
during compilation of your code.
Overloaded Methods
Specifying the actual callback signature is important in case of
overloaded methods. Imagine a class:
package x.y.z;
class Handler {
int pulse() {
return 1;
}
int pulse(int howMuch) {
return howMuch;
}
int pulse(long evenMore) {
return (int) (5 + evenMore);
}
}
you then need to choose in {@link net.java.html.js.JavaScriptBody}
the appropriate method to call:
{@code @}{@link net.java.html.js.JavaScriptBody}(args = { "h" }, javacall = true, // you want to process the @ syntax
body = "return [email protected]::pulse()() +" + // the call to no argument method
"[email protected]::pulse(I)(10) +" + // the call to method with integer argument
"[email protected]::pulse(J)(10);" // the call to method with long argument
)
static native void threePulsesFromJavaScript(Handler h);
static {
assert 26 == threePulsesFromJavaScript(new Handler());
}
To avoid ambiguity, the specification of the correct signature is
required on every call. However, to simplify the development,
there is an annotation processor to
verify the signature really refers to an existing method.
Arrays by Copy
It is possible to exchange arrays between Java and JavaScript. Some
implementations can pass arrays by reference, however in some systems
this is hard to achieve. To choose the least common denominator,
the TCK for behavior of {@link net.java.html.js.JavaScriptBody} requires
the arrays to be always transfered by a copy. As such following code:
{@code @}{@link net.java.html.js.JavaScriptBody}(args = {"arr"}, body = "arr[0] = null;")
private static native void uselessModify(String[] arr);
public static void main(String... args) {
String[] hello = { "Hello", "World!" };
uselessModify(arr);
System.out.println(arr[0] + " " + arr[1]);
}
will still print Hello World! in spite the JavaScript code
sets the 0-th array element to null. Because the array
is passed as a copy, such assignment has no effect on the Java array.
In case one needs to modify an array in a JavaScript and use its
values in Java, one has to return the array back as a return value:
{@code @}{@link net.java.html.js.JavaScriptBody}(args = {"arr"}, body = "arr[0] = 'Ahoy'; return arr;")
private static native Object[] usefulModify(String[] arr);
public static void main(String... args) {
String[] hello = { "Hello", "World!" };
Object[] ret = usefulModify(arr);
System.out.println(ret[0] + " " + ret[1]);
}
now the program prints Ahoy World! as the modified array
is returned back and converted (by a copy) into a Java Object[]
(but of course the ret != hello). Usually the copy based
passing of arrays works OK. It is however good to keep it in mind to
avoid unwanted surprises.
Instance Reference to JavaScript Object
When writing wrappers around existing JavaScript libraries, it may be
useful to hold a reference to some JavaScript object from a Java
instance and use it later.
class WrapperAroundJsObj {
private final Object js;
WrapperAroundJsObj() {
js = initValue();
}
public void set(int v) {
setValue(js, v);
}
{@link net.java.html.js.JavaScriptBody @JavaScriptBody}(args = {}, body = "return { value : 0 };")
private static native Object initValue();
{@link net.java.html.js.JavaScriptBody @JavaScriptBody}(
args = { "js", "v" }, body = "js.value = v;", wait4js = false
)
private static native void setValue(Object js, int v);
}
The type of the Java reference is {@link java.lang.Object}.
From a Java perspective it has no additional methods or fields, however
its properties can be manipulated from JavaScript. Send the object back
to JavaScript by passing it as a parameter of some method
(like the setValue one) and perform necessary JavaScript
calls or changes on it.
Post Process Classes
Classes with {@link net.java.html.js.JavaScriptBody} annotated methods need to
be post processed before they can be used - e.g. their native
body needs to be generated to call into JavaScript (btw. the code is performed
via {@link org.netbeans.html.boot.spi.Fn}). There are three ways
such post processing can happen.
Compile time processing - this is the preferred method that
most of the Html Java APIs are using.
Just include following plugin configuration into your pom.xml
and your classes will be ready for execution as soon as process-classes
Maven phase is over:
<plugin>
<groupId>org.netbeans.html</groupId>
<artifactId>html4j-maven-plugin</artifactId>
<version>${net.java.html.version}</version>
<executions>
<execution>
<id>js-classes</id>
<goals>
<goal>process-js-annotations</goal>
</goals>
</execution>
</executions>
</plugin>
This plugin works in orchestration with
annotation
processor associated with {@link net.java.html.js.JavaScriptBody}
and {@link net.java.html.js.JavaScriptResource} - the processor creates
list of files that need post-processing. The
Maven
plugin reads these files, processes classes mentioned in them and
modifies (and deletes at the end) the files to not include classes
already processed.
Instrumentation Agent - one can do processing in runtime
using JDK's {@link java.lang.instrument.ClassFileTransformer instrumentation}
abilities. The JAR artifact of org.netbeans.html:net.java.html.boot
contains an Agent-Class and Premain-Class
definitions in its manifest. As such one can launch the Java virtual
machine with
$ java -javaagent:jarpath=net.java.html.boot-x.y.jar
and the runtime will take care of processing bytecode of classes
not yet processed in compile time before they are loaded into the
virtual machine.
Special classloading - when booting your application with
{@link net.java.html.boot.BrowserBuilder} there is a 3rd option of
processing the classes. If there are some classes not yet processed
(remember the files listing them generated by the
annotation
processor), the {@link net.java.html.boot.BrowserBuilder#showAndWait() launching method}
will create a special classloader to that does the processing before
loading the bytecode into the virtual machine.
The options are rich, however to avoid any troubles (as the runtime
processing needs to also include asm-5.0.jar on application
classpath), it is recommended
to perform the compile time processing.
Getting Started
There are many ways to start developing
Html for Java application.
However to be sure one chooses the most recent setup, it is recommended
to switch to good old command line and use a
Maven archetype
associated with every version of this project. Just type:
$ mvn archetype:generate \
-DarchetypeGroupId=org.apidesign.html \
-DarchetypeArtifactId=knockout4j-archetype \
-DarchetypeVersion=x.y
Answer few questions (for example choose myfirstbrwsrpage as artifactId) and then you can:
$ cd myfirstbrwsrpage
$ mvn process-classes exec:java
In a few seconds (or minutes if
Maven
decides to download the whole Internet of dependencies) you should
see a sample Hello World application. It is basically composed from one
Java and one HTML file:
$ ls src/main/java/**/DataModel.java
$ ls src/main/webapp/pages/index.html
Play with them, modify them and enjoy
Html for Java!
Mixed Java/JavaScript Debugging
The following video shows how easy it is to use
NetBeans 8.0, JDK8 to debug an application that intermixes Java
and JavaScript calls. One can put breakpoints into Java part,
as well as JavaScript source code, inspect Java as well
as JavaScript variables and switch between these two
languages without any restrictions.
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