<?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="${pmd.website.baseurl}/pmd_rules_java_multithreading.html#avoidsynchronizedatmethodlevel">
<description>
Method-level synchronization can cause problems when new code is added to the method.
Block-level synchronization helps to ensure that only the code that needs synchronization
gets it.
</description>
<priority>3</priority>
<properties>
<property name="xpath">
<value>//MethodDeclaration[@Synchronized='true']</value>
</property>
</properties>
<example>
<![CDATA[
public class Foo {
// Try to avoid this:
synchronized void foo() {
}
// Prefer this:
void bar() {
synchronized(this) {
}
}
// Try to avoid this for static methods:
static synchronized void fooStatic() {
}
// Prefer this:
static void barStatic() {
synchronized(Foo.class) {
}
}
}
]]>
</example>
</rule>
<rule name="AvoidThreadGroup"
language="java"
since="3.6"
message="Avoid using java.lang.ThreadGroup; it is not thread safe"
class="net.sourceforge.pmd.lang.rule.XPathRule"
typeResolution="true"
externalInfoUrl="${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[
//AllocationExpression/ClassOrInterfaceType[pmd-java:typeIs('java.lang.ThreadGroup')]|
//PrimarySuffix[contains(@Image, '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="${pmd.website.baseurl}/pmd_rules_java_multithreading.html#avoidusingvolatile">
<description>
Use of the keyword 'volatile' is generally used to fine tune a Java application, and therefore, requires
a good expertise of the Java Memory Model. Moreover, its range of action is somewhat misknown. Therefore,
the volatile keyword should not be used for maintenance purpose and portability.
</description>
<priority>2</priority>
<properties>
<property name="xpath">
<value>//FieldDeclaration[contains(@Volatile,'true')]</value>
</property>
</properties>
<example>
<![CDATA[
public class ThrDeux {
private volatile String var1; // not suggested
private String var2; // preferred
}
]]>
</example>
</rule>
<rule name="DoNotUseThreads"
language="java"
since="4.1"
class="net.sourceforge.pmd.lang.rule.XPathRule"
message="To be compliant to J2EE, a webapp should not use any thread."
externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_multithreading.html#donotusethreads">
<description>
The J2EE specification explicitly forbids the use of threads.
</description>
<priority>3</priority>
<properties>
<property name="xpath">
<value>//ClassOrInterfaceType[@Image = 'Thread' or @Image = 'Runnable']</value>
</property>
</properties>
<example>
<![CDATA[
// This is not allowed
public class UsingThread extends Thread {
}
// Neither this,
public class OtherThread implements Runnable {
// Nor this ...
public void methode() {
Runnable thread = new Thread(); thread.run();
}
}
]]>
</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"
typeResolution="true"
externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_multithreading.html#dontcallthreadrun">
<description>
Explicitly calling Thread.run() method will execute in the caller's thread of control. Instead, call Thread.start() for the intended behavior.
</description>
<priority>4</priority>
<properties>
<property name="xpath">
<value>
<![CDATA[
//StatementExpression/PrimaryExpression
[
PrimaryPrefix
[
./Name[ends-with(@Image, '.run') or @Image = 'run']
and substring-before(Name/@Image, '.') =//VariableDeclarator/VariableDeclaratorId/@Image
[../../../Type/ReferenceType/ClassOrInterfaceType[pmd-java:typeIs('java.lang.Thread')]]
or (./AllocationExpression/ClassOrInterfaceType[pmd-java:typeIs('java.lang.Thread')]
and ../PrimarySuffix[@Image = 'run'])
]
]
]]>
</value>
</property>
</properties>
<example>
<![CDATA[
Thread t = new Thread();
t.run(); // use t.start() instead
new Thread().run(); // same violation
]]>
</example>
</rule>
<rule name="DoubleCheckedLocking"
language="java"
since="1.04"
message="Double checked locking is not thread safe in Java."
class="net.sourceforge.pmd.lang.java.rule.multithreading.DoubleCheckedLockingRule"
externalInfoUrl="${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"
since="3.4"
message="Singleton is not thread safe"
class="net.sourceforge.pmd.lang.java.rule.multithreading.NonThreadSafeSingletonRule"
externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_multithreading.html#nonthreadsafesingleton">
<description>
Non-thread safe singletons can result in bad state changes. Eliminate
static singletons if possible by instantiating the object directly. Static
singletons are usually not needed as only a single instance exists anyway.
Other possible fixes are to synchronize the entire method or to use an
[initialize-on-demand holder class](https://en.wikipedia.org/wiki/Initialization-on-demand_holder_idiom).
Refrain from using the double-checked locking pattern. The Java Memory Model doesn't
guarantee it to work unless the variable is declared as `volatile`, adding an uneeded
performance penalty. [Reference](http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html)
See Effective Java, item 48.
</description>
<priority>3</priority>
<example>
<![CDATA[
private static Foo foo = null;
//multiple simultaneous callers may see partially initialized objects
public static Foo getFoo() {
if (foo==null) {
foo = new Foo();
}
return foo;
}
]]>
</example>
</rule>
<rule name="UnsynchronizedStaticDateFormatter"
since="3.6"
deprecated="true"
message="Static DateFormatter objects should be accessed in a synchronized manner"
class="net.sourceforge.pmd.lang.java.rule.multithreading.UnsynchronizedStaticDateFormatterRule"
externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_multithreading.html#unsynchronizedstaticdateformatter">
<description>
SimpleDateFormat instances are not synchronized. Sun recommends using separate format instances
for each thread. If multiple threads must access a static formatter, the formatter must be
synchronized either on method or block level.
This rule has been deprecated in favor of the rule {% rule UnsynchronizedStaticFormatter %}.
</description>
<priority>3</priority>
<example>
<![CDATA[
public class Foo {
private static final SimpleDateFormat sdf = new SimpleDateFormat();
void bar() {
sdf.format(); // poor, no thread-safety
}
synchronized void foo() {
sdf.format(); // preferred
}
}
]]>
</example>
</rule>
<rule name="UnsynchronizedStaticFormatter"
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="${pmd.website.baseurl}/pmd_rules_java_multithreading.html#unsynchronizedstaticformatter">
<description>
Instances of `java.text.Format` are generally not synchronized.
Sun recommends using separate format instances for each thread.
If multiple threads must access a static formatter, the formatter must be
synchronized either on method or block level.
</description>
<priority>3</priority>
<example>
<![CDATA[
public class Foo {
private static final SimpleDateFormat sdf = new SimpleDateFormat();
void bar() {
sdf.format(); // poor, no thread-safety
}
synchronized void foo() {
sdf.format(); // preferred
}
}
]]>
</example>
</rule>
<rule name="UseConcurrentHashMap"
language="java"
minimumLanguageVersion="1.5"
since="4.2.6"
message="If you run in Java5 or newer and have concurrent access, you should use the ConcurrentHashMap implementation"
class="net.sourceforge.pmd.lang.rule.XPathRule"
externalInfoUrl="${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[
//Type[../VariableDeclarator/VariableInitializer//AllocationExpression/ClassOrInterfaceType[@Image != 'ConcurrentHashMap']]
/ReferenceType/ClassOrInterfaceType[@Image = 'Map']
]]>
</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[
//StatementExpression/PrimaryExpression
[PrimarySuffix/Arguments[@ArgumentCount = '0']]
[
PrimaryPrefix[
./Name[@Image='notify' or ends-with(@Image,'.notify')]
or ../PrimarySuffix/@Image='notify'
or (./AllocationExpression and ../PrimarySuffix[@Image='notify'])
]
]
]]>
</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>