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<?xml version="1.0"?>
<ruleset name="Multithreading"
xmlns="http://pmd.sourceforge.net/ruleset/2.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://pmd.sourceforge.net/ruleset/2.0.0 https://pmd.sourceforge.io/ruleset_2_0_0.xsd">
<description>
Rules that flag issues when dealing with multiple threads of execution.
</description>
<rule name="AvoidSynchronizedAtMethodLevel"
language="java"
since="3.0"
message="Use block level rather than method level synchronization"
class="net.sourceforge.pmd.lang.rule.XPathRule"
externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_multithreading.html#avoidsynchronizedatmethodlevel">
<description>
Method-level synchronization can cause problems when new code is added to the method.
Block-level synchronization helps to ensure that only the code that needs synchronization
gets it.
</description>
<priority>3</priority>
<properties>
<property name="xpath">
<value>//MethodDeclaration[pmd-java:modifiers() = "synchronized"]</value>
</property>
</properties>
<example>
<![CDATA[
public class Foo {
// Try to avoid this:
synchronized void foo() {
// code, that doesn't need synchronization
// ...
// code, that requires synchronization
if (!sharedData.has("bar")) {
sharedData.add("bar");
}
// more code, that doesn't need synchronization
// ...
}
// Prefer this:
void bar() {
// code, that doesn't need synchronization
// ...
synchronized(this) {
if (!sharedData.has("bar")) {
sharedData.add("bar");
}
}
// more code, that doesn't need synchronization
// ...
}
// Try to avoid this for static methods:
static synchronized void fooStatic() {
}
// Prefer this:
static void barStatic() {
// code, that doesn't need synchronization
// ...
synchronized(Foo.class) {
// code, that requires synchronization
}
// more code, that doesn't need synchronization
// ...
}
}
]]>
</example>
</rule>
<rule name="AvoidThreadGroup"
language="java"
since="3.6"
message="Avoid using java.lang.ThreadGroup; it is not thread safe"
class="net.sourceforge.pmd.lang.rule.XPathRule"
externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_multithreading.html#avoidthreadgroup">
<description>
Avoid using java.lang.ThreadGroup; although it is intended to be used in a threaded environment
it contains methods that are not thread-safe.
</description>
<priority>3</priority>
<properties>
<property name="xpath">
<value>
<![CDATA[
//ConstructorCall/ClassOrInterfaceType[pmd-java:typeIs('java.lang.ThreadGroup')]
| //MethodCall[@MethodName = 'getThreadGroup']
]]>
</value>
</property>
</properties>
<example>
<![CDATA[
public class Bar {
void buz() {
ThreadGroup tg = new ThreadGroup("My threadgroup");
tg = new ThreadGroup(tg, "my thread group");
tg = Thread.currentThread().getThreadGroup();
tg = System.getSecurityManager().getThreadGroup();
}
}
]]>
</example>
</rule>
<rule name="AvoidUsingVolatile"
language="java"
since="4.1"
class="net.sourceforge.pmd.lang.rule.XPathRule"
message="Use of modifier volatile is not recommended."
externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_multithreading.html#avoidusingvolatile">
<description>
Use of the keyword 'volatile' is generally used to fine tune a Java application, and therefore, requires
a good expertise of the Java Memory Model. Moreover, its range of action is somewhat misknown. Therefore,
the volatile keyword should not be used for maintenance purpose and portability.
</description>
<priority>2</priority>
<properties>
<property name="xpath">
<value>//FieldDeclaration[pmd-java:modifiers() = "volatile"]</value>
</property>
</properties>
<example>
<![CDATA[
public class ThrDeux {
private volatile String var1; // not suggested
private String var2; // preferred
}
]]>
</example>
</rule>
<rule name="DoNotUseThreads"
language="java"
since="4.1"
class="net.sourceforge.pmd.lang.rule.XPathRule"
message="To be compliant to J2EE, a webapp should not use any thread."
externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_multithreading.html#donotusethreads">
<description>
The J2EE specification explicitly forbids the use of threads. Threads are resources, that should be managed and monitored by the J2EE server.
If the application creates threads on its own or uses own custom thread pools, then these threads are not managed, which could lead to resource exhaustion.
Also, EJBs might be moved between machines in a cluster and only managed resources can be moved along.
</description>
<priority>3</priority>
<properties>
<property name="xpath">
<value>
<![CDATA[
//ClassOrInterfaceType
[pmd-java:typeIs('java.lang.Thread') or pmd-java:typeIs('java.util.concurrent.ExecutorService')]
(: allow Thread.currentThread().getContextClassLoader() :)
[not(parent::TypeExpression[parent::MethodCall[pmd-java:matchesSig('_#currentThread()')
and parent::MethodCall[pmd-java:matchesSig('_#getContextClassLoader()')]
]
]
)]
(: exclude duplicated types on the same line :)
[not((parent::FieldDeclaration|parent::LocalVariableDeclaration)/VariableDeclarator/*[2][pmd-java:typeIs('java.lang.Thread') or pmd-java:typeIs('java.util.concurrent.ExecutorService')])
or
@BeginLine != (parent::FieldDeclaration|parent::LocalVariableDeclaration)/VariableDeclarator/ConstructorCall/ClassOrInterfaceType/@BeginLine]
|
//MethodCall[*[1][not(pmd-java:nodeIs('MethodCall'))][pmd-java:nodeIs('Expression') and (pmd-java:typeIs('java.util.concurrent.Executors')
or pmd-java:typeIs('java.util.concurrent.ExecutorService'))]]
]]>
</value>
</property>
</properties>
<example>
<![CDATA[
// This is not allowed
public class UsingThread extends Thread {
}
// Neither this,
public class UsingExecutorService {
public void methodX() {
ExecutorService executorService = Executors.newFixedThreadPool(5);
}
}
// Nor this,
public class Example implements ExecutorService {
}
// Nor this,
public class Example extends AbstractExecutorService {
}
// Nor this
public class UsingExecutors {
public void methodX() {
Executors.newSingleThreadExecutor().submit(() -> System.out.println("Hello!"));
}
}
]]>
</example>
</rule>
<rule name="DontCallThreadRun"
language="java"
since="4.3"
message="Don't call Thread.run() explicitly, use Thread.start()"
class="net.sourceforge.pmd.lang.rule.XPathRule"
externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_multithreading.html#dontcallthreadrun">
<description>
Explicitly calling Thread.run() method will execute in the caller's thread of control. Instead, call Thread.start() for the intended behavior.
</description>
<priority>4</priority>
<properties>
<property name="xpath">
<value>
<![CDATA[
//MethodCall[ pmd-java:matchesSig("java.lang.Thread#run()") ]
]]>
</value>
</property>
</properties>
<example>
<![CDATA[
Thread t = new Thread();
t.run(); // use t.start() instead
new Thread().run(); // same violation
]]>
</example>
</rule>
<rule name="DoubleCheckedLocking"
language="java"
since="1.04"
message="Double checked locking is not thread safe in Java."
class="net.sourceforge.pmd.lang.java.rule.multithreading.DoubleCheckedLockingRule"
externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_multithreading.html#doublecheckedlocking">
<description>
Partially created objects can be returned by the Double Checked Locking pattern when used in Java.
An optimizing JRE may assign a reference to the baz variable before it calls the constructor of the object the
reference points to.
Note: With Java 5, you can make Double checked locking work, if you declare the variable to be `volatile`.
For more details refer to: <http://www.javaworld.com/javaworld/jw-02-2001/jw-0209-double.html>
or <http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html>
</description>
<priority>1</priority>
<example>
<![CDATA[
public class Foo {
/*volatile */ Object baz = null; // fix for Java5 and later: volatile
Object bar() {
if (baz == null) { // baz may be non-null yet not fully created
synchronized(this) {
if (baz == null) {
baz = new Object();
}
}
}
return baz;
}
}
]]>
</example>
</rule>
<rule name="NonThreadSafeSingleton"
language="java"
since="3.4"
message="Singleton is not thread safe"
class="net.sourceforge.pmd.lang.java.rule.multithreading.NonThreadSafeSingletonRule"
externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_multithreading.html#nonthreadsafesingleton">
<description>
Non-thread safe singletons can result in bad state changes. Eliminate
static singletons if possible by instantiating the object directly. Static
singletons are usually not needed as only a single instance exists anyway.
Other possible fixes are to synchronize the entire method or to use an
[initialize-on-demand holder class](https://en.wikipedia.org/wiki/Initialization-on-demand_holder_idiom).
Refrain from using the double-checked locking pattern. The Java Memory Model doesn't
guarantee it to work unless the variable is declared as `volatile`, adding an uneeded
performance penalty. [Reference](http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html)
See Effective Java, item 48.
</description>
<priority>3</priority>
<example>
<![CDATA[
private static Foo foo = null;
//multiple simultaneous callers may see partially initialized objects
public static Foo getFoo() {
if (foo==null) {
foo = new Foo();
}
return foo;
}
]]>
</example>
</rule>
<rule name="UnsynchronizedStaticFormatter"
language="java"
since="6.11.0"
message="Static Formatter objects should be accessed in a synchronized manner"
class="net.sourceforge.pmd.lang.java.rule.multithreading.UnsynchronizedStaticFormatterRule"
externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_multithreading.html#unsynchronizedstaticformatter">
<description>
Instances of `java.text.Format` are generally not synchronized.
Sun recommends using separate format instances for each thread.
If multiple threads must access a static formatter, the formatter must be
synchronized on block level.
</description>
<priority>3</priority>
<example>
<![CDATA[
public class Foo {
private static final SimpleDateFormat sdf = new SimpleDateFormat();
void bar() {
sdf.format(); // poor, no thread-safety
}
void foo() {
synchronized (sdf) { // preferred
sdf.format();
}
}
}
]]>
</example>
</rule>
<rule name="UseConcurrentHashMap"
language="java"
minimumLanguageVersion="1.5"
since="4.2.6"
message="If you run in Java5 or newer and have concurrent access, you should use the ConcurrentHashMap implementation"
class="net.sourceforge.pmd.lang.rule.XPathRule"
externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_multithreading.html#useconcurrenthashmap">
<description>
Since Java5 brought a new implementation of the Map designed for multi-threaded access, you can
perform efficient map reads without blocking other threads.
</description>
<priority>3</priority>
<properties>
<property name="xpath">
<value>
<![CDATA[
//VariableDeclarator[VariableDeclaratorId[pmd-java:typeIsExactly('java.util.Map')] and *[2][self::ConstructorCall and not(pmd-java:typeIs('java.util.concurrent.ConcurrentHashMap'))]]
]]>
</value>
</property>
</properties>
<example>
<![CDATA[
public class ConcurrentApp {
public void getMyInstance() {
Map map1 = new HashMap(); // fine for single-threaded access
Map map2 = new ConcurrentHashMap(); // preferred for use with multiple threads
// the following case will be ignored by this rule
Map map3 = someModule.methodThatReturnMap(); // might be OK, if the returned map is already thread-safe
}
}
]]>
</example>
</rule>
<rule name="UseNotifyAllInsteadOfNotify"
language="java"
since="3.0"
message="Call Thread.notifyAll() rather than Thread.notify()"
class="net.sourceforge.pmd.lang.rule.XPathRule"
externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_multithreading.html#usenotifyallinsteadofnotify">
<description>
Thread.notify() awakens a thread monitoring the object. If more than one thread is monitoring, then only
one is chosen. The thread chosen is arbitrary; thus its usually safer to call notifyAll() instead.
</description>
<priority>3</priority>
<properties>
<property name="xpath">
<value>
<![CDATA[
//MethodCall[@MethodName="notify" and ArgumentList[count(*) = 0]]
]]>
</value>
</property>
</properties>
<example>
<![CDATA[
void bar() {
x.notify();
// If many threads are monitoring x, only one (and you won't know which) will be notified.
// use instead:
x.notifyAll();
}
]]>
</example>
</rule>
</ruleset>
