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» Multithreading
<a
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<h1 class="post-title-main">Multithreading</h1>
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<div class="post-content" data-github-edit-url="https://github.com/pmd/pmd/blob/main/docs/../pmd-java/src/main/resources/category/java/multithreading.xml">
<div class="summary">Rules that flag issues when dealing with multiple threads of execution.</div>
<details id="inline-toc-details">
<summary>Table of Contents</summary>
<div id="inline-toc"><!-- empty, move TOC here when screen size too small --></div>
</details>
<!-- DO NOT EDIT THIS FILE. This file is generated from file ../pmd-java/src/main/resources/category/java/multithreading.xml. -->
<h2 id="avoidsynchronizedatmethodlevel">AvoidSynchronizedAtMethodLevel</h2>
<p><strong>Since:</strong> PMD 3.0</p>
<p><strong>Priority:</strong> Medium (3)</p>
<p>Method-level synchronization will pin virtual threads and can cause performance problems. Additionally, it can cause
problems when new code is added to the method. Block-level ReentrantLock helps to ensure that only the code that
needs mutual exclusion will be locked.</p>
<p><strong>This rule is defined by the following XPath expression:</strong></p>
<div class="language-xpath highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="o">//</span><span class="nt">MethodDeclaration</span><span class="p">[</span><span class="nn">pmd-java:</span><span class="ow">mod</span><span class="kr">if</span><span class="nf">iers</span><span class="p">()</span><span class="w"> </span><span class="err">=</span><span class="w"> </span><span class="s2">"synchronized"</span><span class="p">]</span><span class="w">
</span></code></pre></div></div>
<p><strong>Example(s):</strong></p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">public</span> <span class="kd">class</span> <span class="nc">Foo</span> <span class="o">{</span>
<span class="c1">// Try to avoid this:</span>
<span class="kd">synchronized</span> <span class="kt">void</span> <span class="nf">foo</span><span class="o">()</span> <span class="o">{</span>
<span class="c1">// code, that doesn't need synchronization</span>
<span class="c1">// ...</span>
<span class="c1">// code, that requires synchronization</span>
<span class="k">if</span> <span class="o">(!</span><span class="n">sharedData</span><span class="o">.</span><span class="na">has</span><span class="o">(</span><span class="s">"bar"</span><span class="o">))</span> <span class="o">{</span>
<span class="n">sharedData</span><span class="o">.</span><span class="na">add</span><span class="o">(</span><span class="s">"bar"</span><span class="o">);</span>
<span class="o">}</span>
<span class="c1">// more code, that doesn't need synchronization</span>
<span class="c1">// ...</span>
<span class="o">}</span>
<span class="c1">// Prefer this:</span>
<span class="nc">Lock</span> <span class="n">instanceLock</span> <span class="o">=</span> <span class="k">new</span> <span class="nc">ReentrantLock</span><span class="o">();</span>
<span class="kt">void</span> <span class="nf">bar</span><span class="o">()</span> <span class="o">{</span>
<span class="c1">// code, that doesn't need synchronization</span>
<span class="c1">// ...</span>
<span class="k">try</span> <span class="o">{</span>
<span class="n">instanceLock</span><span class="o">.</span><span class="na">lock</span><span class="o">();</span> <span class="c1">// or instanceLock.tryLock(long time, TimeUnit unit)</span>
<span class="k">if</span> <span class="o">(!</span><span class="n">sharedData</span><span class="o">.</span><span class="na">has</span><span class="o">(</span><span class="s">"bar"</span><span class="o">))</span> <span class="o">{</span>
<span class="n">sharedData</span><span class="o">.</span><span class="na">add</span><span class="o">(</span><span class="s">"bar"</span><span class="o">);</span>
<span class="o">}</span>
<span class="o">}</span> <span class="k">finally</span> <span class="o">{</span>
<span class="n">instanceLock</span><span class="o">.</span><span class="na">unlock</span><span class="o">();</span>
<span class="o">}</span>
<span class="c1">// more code, that doesn't need synchronization</span>
<span class="c1">// ...</span>
<span class="o">}</span>
<span class="c1">// Try to avoid this for static methods:</span>
<span class="kd">static</span> <span class="kd">synchronized</span> <span class="kt">void</span> <span class="nf">fooStatic</span><span class="o">()</span> <span class="o">{</span>
<span class="o">}</span>
<span class="c1">// Prefer this:</span>
<span class="kd">private</span> <span class="kd">static</span> <span class="nc">Lock</span> <span class="no">CLASS_LOCK</span> <span class="o">=</span> <span class="k">new</span> <span class="nc">ReentrantLock</span><span class="o">();</span>
<span class="kd">static</span> <span class="kt">void</span> <span class="nf">barStatic</span><span class="o">()</span> <span class="o">{</span>
<span class="c1">// code, that doesn't need synchronization</span>
<span class="c1">// ...</span>
<span class="k">try</span> <span class="o">{</span>
<span class="no">CLS_LOCK</span><span class="o">.</span><span class="na">lock</span><span class="o">();</span>
<span class="c1">// code, that requires synchronization</span>
<span class="o">}</span> <span class="k">finally</span> <span class="o">{</span>
<span class="no">CLS_LOCK</span><span class="o">.</span><span class="na">unlock</span><span class="o">();</span>
<span class="o">}</span>
<span class="c1">// more code, that doesn't need synchronization</span>
<span class="c1">// ...</span>
<span class="o">}</span>
<span class="o">}</span>
</code></pre></div></div>
<p><strong>Use this rule by referencing it:</strong></p>
<div class="language-xml highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nt">&lt;rule</span> <span class="na">ref=</span><span class="s">"category/java/multithreading.xml/AvoidSynchronizedAtMethodLevel"</span> <span class="nt">/&gt;</span>
</code></pre></div></div>
<h2 id="avoidsynchronizedstatement">AvoidSynchronizedStatement</h2>
<p><strong>Since:</strong> PMD 7.5.0</p>
<p><strong>Priority:</strong> Medium (3)</p>
<p>Synchronization will pin virtual threads and can cause performance problems.</p>
<p><strong>This rule is defined by the following XPath expression:</strong></p>
<div class="language-xpath highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="o">//</span><span class="nt">SynchronizedStatement</span><span class="w">
</span></code></pre></div></div>
<p><strong>Example(s):</strong></p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">public</span> <span class="kd">class</span> <span class="nc">Foo</span> <span class="o">{</span>
<span class="c1">// Try to avoid this:</span>
<span class="kt">void</span> <span class="nf">foo</span><span class="o">()</span> <span class="o">{</span>
<span class="c1">// code that doesn't need mutual exclusion</span>
<span class="kd">synchronized</span><span class="o">(</span><span class="k">this</span><span class="o">)</span> <span class="o">{</span>
<span class="c1">// code that requires mutual exclusion</span>
<span class="o">}</span>
<span class="c1">// more code that doesn't need mutual exclusion</span>
<span class="o">}</span>
<span class="c1">// Prefer this:</span>
<span class="nc">Lock</span> <span class="n">instanceLock</span> <span class="o">=</span> <span class="k">new</span> <span class="nc">ReentrantLock</span><span class="o">();</span>
<span class="kt">void</span> <span class="nf">foo</span><span class="o">()</span> <span class="o">{</span>
<span class="c1">// code that doesn't need mutual exclusion</span>
<span class="k">try</span> <span class="o">{</span>
<span class="n">instanceLock</span><span class="o">.</span><span class="na">lock</span><span class="o">();</span> <span class="c1">// or instanceLock.tryLock(long time, TimeUnit unit)</span>
<span class="c1">// code that requires mutual exclusion</span>
<span class="o">}</span> <span class="k">finally</span> <span class="o">{</span>
<span class="n">instanceLock</span><span class="o">.</span><span class="na">unlock</span><span class="o">();</span>
<span class="o">}</span>
<span class="c1">// more code that doesn't need mutual exclusion</span>
<span class="o">}</span>
<span class="o">}</span>
</code></pre></div></div>
<p><strong>Use this rule by referencing it:</strong></p>
<div class="language-xml highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nt">&lt;rule</span> <span class="na">ref=</span><span class="s">"category/java/multithreading.xml/AvoidSynchronizedStatement"</span> <span class="nt">/&gt;</span>
</code></pre></div></div>
<h2 id="avoidthreadgroup">AvoidThreadGroup</h2>
<p><strong>Since:</strong> PMD 3.6</p>
<p><strong>Priority:</strong> Medium (3)</p>
<p>Avoid using java.lang.ThreadGroup; although it is intended to be used in a threaded environment
it contains methods that are not thread-safe.</p>
<p><strong>This rule is defined by the following XPath expression:</strong></p>
<div class="language-xpath highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="o">//</span><span class="nt">ConstructorCall</span><span class="o">/</span><span class="nt">ClassType</span><span class="p">[</span><span class="nn">pmd-java:</span><span class="nf">typeIs</span><span class="p">(</span><span class="s1">'java.lang.ThreadGroup'</span><span class="p">)]</span><span class="w">
</span><span class="p">|</span><span class="w"> </span><span class="o">//</span><span class="nt">MethodCall</span><span class="p">[</span><span class="na">@MethodName</span><span class="w"> </span><span class="err">=</span><span class="w"> </span><span class="s1">'getThreadGroup'</span><span class="p">]</span><span class="w">
</span></code></pre></div></div>
<p><strong>Example(s):</strong></p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">public</span> <span class="kd">class</span> <span class="nc">Bar</span> <span class="o">{</span>
<span class="kt">void</span> <span class="nf">buz</span><span class="o">()</span> <span class="o">{</span>
<span class="nc">ThreadGroup</span> <span class="n">tg</span> <span class="o">=</span> <span class="k">new</span> <span class="nc">ThreadGroup</span><span class="o">(</span><span class="s">"My threadgroup"</span><span class="o">);</span>
<span class="n">tg</span> <span class="o">=</span> <span class="k">new</span> <span class="nc">ThreadGroup</span><span class="o">(</span><span class="n">tg</span><span class="o">,</span> <span class="s">"my thread group"</span><span class="o">);</span>
<span class="n">tg</span> <span class="o">=</span> <span class="nc">Thread</span><span class="o">.</span><span class="na">currentThread</span><span class="o">().</span><span class="na">getThreadGroup</span><span class="o">();</span>
<span class="n">tg</span> <span class="o">=</span> <span class="nc">System</span><span class="o">.</span><span class="na">getSecurityManager</span><span class="o">().</span><span class="na">getThreadGroup</span><span class="o">();</span>
<span class="o">}</span>
<span class="o">}</span>
</code></pre></div></div>
<p><strong>Use this rule by referencing it:</strong></p>
<div class="language-xml highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nt">&lt;rule</span> <span class="na">ref=</span><span class="s">"category/java/multithreading.xml/AvoidThreadGroup"</span> <span class="nt">/&gt;</span>
</code></pre></div></div>
<h2 id="avoidusingvolatile">AvoidUsingVolatile</h2>
<p><strong>Since:</strong> PMD 4.1</p>
<p><strong>Priority:</strong> Medium High (2)</p>
<p>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.</p>
<p><strong>This rule is defined by the following XPath expression:</strong></p>
<div class="language-xpath highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="o">//</span><span class="nt">FieldDeclaration</span><span class="p">[</span><span class="nn">pmd-java:</span><span class="ow">mod</span><span class="kr">if</span><span class="nf">iers</span><span class="p">()</span><span class="w"> </span><span class="err">=</span><span class="w"> </span><span class="s2">"volatile"</span><span class="p">]</span><span class="w">
</span></code></pre></div></div>
<p><strong>Example(s):</strong></p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">public</span> <span class="kd">class</span> <span class="nc">ThrDeux</span> <span class="o">{</span>
<span class="kd">private</span> <span class="kd">volatile</span> <span class="nc">String</span> <span class="n">var1</span><span class="o">;</span> <span class="c1">// not suggested</span>
<span class="kd">private</span> <span class="nc">String</span> <span class="n">var2</span><span class="o">;</span> <span class="c1">// preferred</span>
<span class="o">}</span>
</code></pre></div></div>
<p><strong>Use this rule by referencing it:</strong></p>
<div class="language-xml highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nt">&lt;rule</span> <span class="na">ref=</span><span class="s">"category/java/multithreading.xml/AvoidUsingVolatile"</span> <span class="nt">/&gt;</span>
</code></pre></div></div>
<h2 id="donotusethreads">DoNotUseThreads</h2>
<p><strong>Since:</strong> PMD 4.1</p>
<p><strong>Priority:</strong> Medium (3)</p>
<p>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.</p>
<p><strong>This rule is defined by the following XPath expression:</strong></p>
<div class="language-xpath highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="o">//</span><span class="nt">ClassType</span><span class="w">
</span><span class="p">[</span><span class="nn">pmd-java:</span><span class="nf">typeIs</span><span class="p">(</span><span class="s1">'java.lang.Thread'</span><span class="p">)</span><span class="w"> </span><span class="ow">or</span><span class="w"> </span><span class="nn">pmd-java:</span><span class="nf">typeIs</span><span class="p">(</span><span class="s1">'java.util.concurrent.ExecutorService'</span><span class="p">)]</span><span class="w">
</span><span class="c">(: allow Thread.currentThread().getContextClassLoader() :)</span><span class="w">
</span><span class="p">[</span><span class="ow">not</span><span class="p">(</span><span class="kn">parent::</span><span class="nt">TypeExpression</span><span class="p">[</span><span class="kn">parent::</span><span class="nt">MethodCall</span><span class="p">[</span><span class="nn">pmd-java:</span><span class="nf">matchesSig</span><span class="p">(</span><span class="s1">'_#currentThread()'</span><span class="p">)</span><span class="w">
</span><span class="ow">and</span><span class="w"> </span><span class="kn">parent::</span><span class="nt">MethodCall</span><span class="p">[</span><span class="nn">pmd-java:</span><span class="nf">matchesSig</span><span class="p">(</span><span class="s1">'_#getContextClassLoader()'</span><span class="p">)]</span><span class="w">
</span><span class="p">]</span><span class="w">
</span><span class="p">]</span><span class="w">
</span><span class="p">)]</span><span class="w">
</span><span class="c">(: exclude duplicated types on the same line :)</span><span class="w">
</span><span class="p">[</span><span class="ow">not</span><span class="p">((</span><span class="kn">parent::</span><span class="nt">FieldDeclaration</span><span class="p">|</span><span class="kn">parent::</span><span class="nt">LocalVariableDeclaration</span><span class="p">)</span><span class="o">/</span><span class="nt">VariableDeclarator</span><span class="err">/</span><span class="o">*</span><span class="p">[</span><span class="m">2</span><span class="p">][</span><span class="nn">pmd-java:</span><span class="nf">typeIs</span><span class="p">(</span><span class="s1">'java.lang.Thread'</span><span class="p">)</span><span class="w"> </span><span class="ow">or</span><span class="w"> </span><span class="nn">pmd-java:</span><span class="nf">typeIs</span><span class="p">(</span><span class="s1">'java.util.concurrent.ExecutorService'</span><span class="p">)])</span><span class="w">
</span><span class="ow">or</span><span class="w">
</span><span class="na">@BeginLine</span><span class="w"> </span><span class="p">!</span><span class="err">=</span><span class="w"> </span><span class="p">(</span><span class="kn">parent::</span><span class="nt">FieldDeclaration</span><span class="p">|</span><span class="kn">parent::</span><span class="nt">LocalVariableDeclaration</span><span class="p">)</span><span class="o">/</span><span class="nt">VariableDeclarator</span><span class="o">/</span><span class="nt">ConstructorCall</span><span class="o">/</span><span class="nt">ClassType</span><span class="err">/</span><span class="na">@BeginLine</span><span class="p">]</span><span class="w">
</span><span class="p">|</span><span class="w">
</span><span class="o">//</span><span class="nt">MethodCall</span><span class="p">[</span><span class="o">*</span><span class="p">[</span><span class="m">1</span><span class="p">][</span><span class="ow">not</span><span class="p">(</span><span class="nn">pmd-java:</span><span class="nf">nodeIs</span><span class="p">(</span><span class="s1">'MethodCall'</span><span class="p">))][</span><span class="nn">pmd-java:</span><span class="nf">nodeIs</span><span class="p">(</span><span class="s1">'Expression'</span><span class="p">)</span><span class="w"> </span><span class="ow">and</span><span class="w"> </span><span class="p">(</span><span class="nn">pmd-java:</span><span class="nf">typeIs</span><span class="p">(</span><span class="s1">'java.util.concurrent.Executors'</span><span class="p">)</span><span class="w">
</span><span class="ow">or</span><span class="w"> </span><span class="nn">pmd-java:</span><span class="nf">typeIs</span><span class="p">(</span><span class="s1">'java.util.concurrent.ExecutorService'</span><span class="p">))]]</span><span class="w">
</span></code></pre></div></div>
<p><strong>Example(s):</strong></p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1">// This is not allowed</span>
<span class="kd">public</span> <span class="kd">class</span> <span class="nc">UsingThread</span> <span class="kd">extends</span> <span class="nc">Thread</span> <span class="o">{</span>
<span class="o">}</span>
<span class="c1">// Neither this,</span>
<span class="kd">public</span> <span class="kd">class</span> <span class="nc">UsingExecutorService</span> <span class="o">{</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">methodX</span><span class="o">()</span> <span class="o">{</span>
<span class="nc">ExecutorService</span> <span class="n">executorService</span> <span class="o">=</span> <span class="nc">Executors</span><span class="o">.</span><span class="na">newFixedThreadPool</span><span class="o">(</span><span class="mi">5</span><span class="o">);</span>
<span class="o">}</span>
<span class="o">}</span>
<span class="c1">// Nor this,</span>
<span class="kd">public</span> <span class="kd">class</span> <span class="nc">Example</span> <span class="kd">implements</span> <span class="nc">ExecutorService</span> <span class="o">{</span>
<span class="o">}</span>
<span class="c1">// Nor this,</span>
<span class="kd">public</span> <span class="kd">class</span> <span class="nc">Example</span> <span class="kd">extends</span> <span class="nc">AbstractExecutorService</span> <span class="o">{</span>
<span class="o">}</span>
<span class="c1">// Nor this</span>
<span class="kd">public</span> <span class="kd">class</span> <span class="nc">UsingExecutors</span> <span class="o">{</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">methodX</span><span class="o">()</span> <span class="o">{</span>
<span class="nc">Executors</span><span class="o">.</span><span class="na">newSingleThreadExecutor</span><span class="o">().</span><span class="na">submit</span><span class="o">(()</span> <span class="o">-&gt;</span> <span class="nc">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">(</span><span class="s">"Hello!"</span><span class="o">));</span>
<span class="o">}</span>
<span class="o">}</span>
</code></pre></div></div>
<p><strong>Use this rule by referencing it:</strong></p>
<div class="language-xml highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nt">&lt;rule</span> <span class="na">ref=</span><span class="s">"category/java/multithreading.xml/DoNotUseThreads"</span> <span class="nt">/&gt;</span>
</code></pre></div></div>
<h2 id="dontcallthreadrun">DontCallThreadRun</h2>
<p><strong>Since:</strong> PMD 4.3</p>
<p><strong>Priority:</strong> Medium Low (4)</p>
<p>Explicitly calling Thread.run() method will execute in the caller’s thread of control. Instead, call Thread.start() for the intended behavior.</p>
<p><strong>This rule is defined by the following XPath expression:</strong></p>
<div class="language-xpath highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="o">//</span><span class="nt">MethodCall</span><span class="p">[</span><span class="w"> </span><span class="nn">pmd-java:</span><span class="nf">matchesSig</span><span class="p">(</span><span class="s2">"java.lang.Thread#run()"</span><span class="p">)</span><span class="w"> </span><span class="p">]</span><span class="w">
</span></code></pre></div></div>
<p><strong>Example(s):</strong></p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nc">Thread</span> <span class="n">t</span> <span class="o">=</span> <span class="k">new</span> <span class="nc">Thread</span><span class="o">();</span>
<span class="n">t</span><span class="o">.</span><span class="na">run</span><span class="o">();</span> <span class="c1">// use t.start() instead</span>
<span class="k">new</span> <span class="nf">Thread</span><span class="o">().</span><span class="na">run</span><span class="o">();</span> <span class="c1">// same violation</span>
</code></pre></div></div>
<p><strong>Use this rule by referencing it:</strong></p>
<div class="language-xml highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nt">&lt;rule</span> <span class="na">ref=</span><span class="s">"category/java/multithreading.xml/DontCallThreadRun"</span> <span class="nt">/&gt;</span>
</code></pre></div></div>
<h2 id="doublecheckedlocking">DoubleCheckedLocking</h2>
<p><strong>Since:</strong> PMD 1.04</p>
<p><strong>Priority:</strong> High (1)</p>
<p>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.</p>
<p>Note: With Java 5, you can make Double checked locking work, if you declare the variable to be <code class="language-plaintext highlighter-rouge">volatile</code>.</p>
<p>For more details refer to: <a href="http://www.javaworld.com/javaworld/jw-02-2001/jw-0209-double.html">http://www.javaworld.com/javaworld/jw-02-2001/jw-0209-double.html</a>
or <a href="http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html">http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html</a></p>
<p><strong>This rule is defined by the following Java class:</strong> <a href="https://github.com/pmd/pmd/blob/main/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/rule/multithreading/DoubleCheckedLockingRule.java">net.sourceforge.pmd.lang.java.rule.multithreading.DoubleCheckedLockingRule</a></p>
<p><strong>Example(s):</strong></p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">public</span> <span class="kd">class</span> <span class="nc">Foo</span> <span class="o">{</span>
<span class="cm">/*volatile */</span> <span class="nc">Object</span> <span class="n">baz</span> <span class="o">=</span> <span class="kc">null</span><span class="o">;</span> <span class="c1">// fix for Java5 and later: volatile</span>
<span class="nc">Object</span> <span class="nf">bar</span><span class="o">()</span> <span class="o">{</span>
<span class="k">if</span> <span class="o">(</span><span class="n">baz</span> <span class="o">==</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span> <span class="c1">// baz may be non-null yet not fully created</span>
<span class="kd">synchronized</span><span class="o">(</span><span class="k">this</span><span class="o">)</span> <span class="o">{</span>
<span class="k">if</span> <span class="o">(</span><span class="n">baz</span> <span class="o">==</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
<span class="n">baz</span> <span class="o">=</span> <span class="k">new</span> <span class="nc">Object</span><span class="o">();</span>
<span class="o">}</span>
<span class="o">}</span>
<span class="o">}</span>
<span class="k">return</span> <span class="n">baz</span><span class="o">;</span>
<span class="o">}</span>
<span class="o">}</span>
</code></pre></div></div>
<p><strong>Use this rule by referencing it:</strong></p>
<div class="language-xml highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nt">&lt;rule</span> <span class="na">ref=</span><span class="s">"category/java/multithreading.xml/DoubleCheckedLocking"</span> <span class="nt">/&gt;</span>
</code></pre></div></div>
<h2 id="nonthreadsafesingleton">NonThreadSafeSingleton</h2>
<p><strong>Since:</strong> PMD 3.4</p>
<p><strong>Priority:</strong> Medium (3)</p>
<p>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
<a href="https://en.wikipedia.org/wiki/Initialization-on-demand_holder_idiom">initialize-on-demand holder class</a>.</p>
<p>Refrain from using the double-checked locking pattern. The Java Memory Model doesn’t
guarantee it to work unless the variable is declared as <code class="language-plaintext highlighter-rouge">volatile</code>, adding an uneeded
performance penalty. <a href="http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html">Reference</a></p>
<p>See Effective Java, item 48.</p>
<p><strong>This rule is defined by the following Java class:</strong> <a href="https://github.com/pmd/pmd/blob/main/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/rule/multithreading/NonThreadSafeSingletonRule.java">net.sourceforge.pmd.lang.java.rule.multithreading.NonThreadSafeSingletonRule</a></p>
<p><strong>Example(s):</strong></p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">private</span> <span class="kd">static</span> <span class="nc">Foo</span> <span class="n">foo</span> <span class="o">=</span> <span class="kc">null</span><span class="o">;</span>
<span class="c1">//multiple simultaneous callers may see partially initialized objects</span>
<span class="kd">public</span> <span class="kd">static</span> <span class="nc">Foo</span> <span class="nf">getFoo</span><span class="o">()</span> <span class="o">{</span>
<span class="k">if</span> <span class="o">(</span><span class="n">foo</span><span class="o">==</span><span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
<span class="n">foo</span> <span class="o">=</span> <span class="k">new</span> <span class="nc">Foo</span><span class="o">();</span>
<span class="o">}</span>
<span class="k">return</span> <span class="n">foo</span><span class="o">;</span>
<span class="o">}</span>
</code></pre></div></div>
<p><strong>This rule has the following properties:</strong></p>
<table>
<thead>
<tr>
<th>Name</th>
<th>Default Value</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td>checkNonStaticMethods</td>
<td>true</td>
<td>Check for non-static methods. Do not set this to false and checkNonStaticFields to true.</td>
</tr>
<tr>
<td>checkNonStaticFields</td>
<td>false</td>
<td>Check for non-static fields. Do not set this to true and checkNonStaticMethods to false.</td>
</tr>
</tbody>
</table>
<p><strong>Use this rule with the default properties by just referencing it:</strong></p>
<div class="language-xml highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nt">&lt;rule</span> <span class="na">ref=</span><span class="s">"category/java/multithreading.xml/NonThreadSafeSingleton"</span> <span class="nt">/&gt;</span>
</code></pre></div></div>
<p><strong>Use this rule and customize it:</strong></p>
<div class="language-xml highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nt">&lt;rule</span> <span class="na">ref=</span><span class="s">"category/java/multithreading.xml/NonThreadSafeSingleton"</span><span class="nt">&gt;</span>
<span class="nt">&lt;properties&gt;</span>
<span class="nt">&lt;property</span> <span class="na">name=</span><span class="s">"checkNonStaticMethods"</span> <span class="na">value=</span><span class="s">"true"</span> <span class="nt">/&gt;</span>
<span class="nt">&lt;property</span> <span class="na">name=</span><span class="s">"checkNonStaticFields"</span> <span class="na">value=</span><span class="s">"false"</span> <span class="nt">/&gt;</span>
<span class="nt">&lt;/properties&gt;</span>
<span class="nt">&lt;/rule&gt;</span>
</code></pre></div></div>
<h2 id="unsynchronizedstaticformatter">UnsynchronizedStaticFormatter</h2>
<p><strong>Since:</strong> PMD 6.11.0</p>
<p><strong>Priority:</strong> Medium (3)</p>
<p>Instances of <code class="language-plaintext highlighter-rouge">java.text.Format</code> 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.</p>
<p><strong>This rule is defined by the following Java class:</strong> <a href="https://github.com/pmd/pmd/blob/main/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/rule/multithreading/UnsynchronizedStaticFormatterRule.java">net.sourceforge.pmd.lang.java.rule.multithreading.UnsynchronizedStaticFormatterRule</a></p>
<p><strong>Example(s):</strong></p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">public</span> <span class="kd">class</span> <span class="nc">Foo</span> <span class="o">{</span>
<span class="kd">private</span> <span class="kd">static</span> <span class="kd">final</span> <span class="nc">SimpleDateFormat</span> <span class="n">sdf</span> <span class="o">=</span> <span class="k">new</span> <span class="nc">SimpleDateFormat</span><span class="o">();</span>
<span class="kt">void</span> <span class="nf">bar</span><span class="o">()</span> <span class="o">{</span>
<span class="n">sdf</span><span class="o">.</span><span class="na">format</span><span class="o">();</span> <span class="c1">// poor, no thread-safety</span>
<span class="o">}</span>
<span class="kt">void</span> <span class="nf">foo</span><span class="o">()</span> <span class="o">{</span>
<span class="kd">synchronized</span> <span class="o">(</span><span class="n">sdf</span><span class="o">)</span> <span class="o">{</span> <span class="c1">// preferred</span>
<span class="n">sdf</span><span class="o">.</span><span class="na">format</span><span class="o">();</span>
<span class="o">}</span>
<span class="o">}</span>
<span class="o">}</span>
</code></pre></div></div>
<p><strong>This rule has the following properties:</strong></p>
<table>
<thead>
<tr>
<th>Name</th>
<th>Default Value</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td>allowMethodLevelSynchronization</td>
<td>false</td>
<td>If true, method level synchronization is allowed as well as synchronized block. Otherwise only synchronized blocks are allowed.</td>
</tr>
</tbody>
</table>
<p><strong>Use this rule with the default properties by just referencing it:</strong></p>
<div class="language-xml highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nt">&lt;rule</span> <span class="na">ref=</span><span class="s">"category/java/multithreading.xml/UnsynchronizedStaticFormatter"</span> <span class="nt">/&gt;</span>
</code></pre></div></div>
<p><strong>Use this rule and customize it:</strong></p>
<div class="language-xml highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nt">&lt;rule</span> <span class="na">ref=</span><span class="s">"category/java/multithreading.xml/UnsynchronizedStaticFormatter"</span><span class="nt">&gt;</span>
<span class="nt">&lt;properties&gt;</span>
<span class="nt">&lt;property</span> <span class="na">name=</span><span class="s">"allowMethodLevelSynchronization"</span> <span class="na">value=</span><span class="s">"false"</span> <span class="nt">/&gt;</span>
<span class="nt">&lt;/properties&gt;</span>
<span class="nt">&lt;/rule&gt;</span>
</code></pre></div></div>
<h2 id="useconcurrenthashmap">UseConcurrentHashMap</h2>
<p><strong>Since:</strong> PMD 4.2.6</p>
<p><strong>Priority:</strong> Medium (3)</p>
<p><strong>Minimum Language Version:</strong> Java 1.5</p>
<p>Since Java5 brought a new implementation of the Map designed for multi-threaded access, you can
perform efficient map reads without blocking other threads.</p>
<p><strong>This rule is defined by the following XPath expression:</strong></p>
<div class="language-xpath highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="o">//</span><span class="nt">VariableDeclarator</span><span class="p">[</span><span class="nt">VariableId</span><span class="p">[</span><span class="nn">pmd-java:</span><span class="nf">typeIsExactly</span><span class="p">(</span><span class="s1">'java.util.Map'</span><span class="p">)]</span><span class="w"> </span><span class="ow">and</span><span class="w"> </span><span class="o">*</span><span class="p">[</span><span class="m">2</span><span class="p">][</span><span class="kn">self::</span><span class="nt">ConstructorCall</span><span class="w"> </span><span class="ow">and</span><span class="w"> </span><span class="ow">not</span><span class="p">(</span><span class="nn">pmd-java:</span><span class="nf">typeIs</span><span class="p">(</span><span class="s1">'java.util.concurrent.ConcurrentHashMap'</span><span class="p">))]]</span><span class="w">
</span></code></pre></div></div>
<p><strong>Example(s):</strong></p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">public</span> <span class="kd">class</span> <span class="nc">ConcurrentApp</span> <span class="o">{</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">getMyInstance</span><span class="o">()</span> <span class="o">{</span>
<span class="nc">Map</span> <span class="n">map1</span> <span class="o">=</span> <span class="k">new</span> <span class="nc">HashMap</span><span class="o">();</span> <span class="c1">// fine for single-threaded access</span>
<span class="nc">Map</span> <span class="n">map2</span> <span class="o">=</span> <span class="k">new</span> <span class="nc">ConcurrentHashMap</span><span class="o">();</span> <span class="c1">// preferred for use with multiple threads</span>
<span class="c1">// the following case will be ignored by this rule</span>
<span class="nc">Map</span> <span class="n">map3</span> <span class="o">=</span> <span class="n">someModule</span><span class="o">.</span><span class="na">methodThatReturnMap</span><span class="o">();</span> <span class="c1">// might be OK, if the returned map is already thread-safe</span>
<span class="o">}</span>
<span class="o">}</span>
</code></pre></div></div>
<p><strong>Use this rule by referencing it:</strong></p>
<div class="language-xml highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nt">&lt;rule</span> <span class="na">ref=</span><span class="s">"category/java/multithreading.xml/UseConcurrentHashMap"</span> <span class="nt">/&gt;</span>
</code></pre></div></div>
<h2 id="usenotifyallinsteadofnotify">UseNotifyAllInsteadOfNotify</h2>
<p><strong>Since:</strong> PMD 3.0</p>
<p><strong>Priority:</strong> Medium (3)</p>
<p>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.</p>
<p><strong>This rule is defined by the following XPath expression:</strong></p>
<div class="language-xpath highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="o">//</span><span class="nt">MethodCall</span><span class="p">[</span><span class="na">@MethodName</span><span class="err">=</span><span class="s2">"notify"</span><span class="w"> </span><span class="ow">and</span><span class="w"> </span><span class="nt">ArgumentList</span><span class="p">[</span><span class="nf">count</span><span class="p">(</span><span class="err">*</span><span class="p">)</span><span class="w"> </span><span class="err">=</span><span class="w"> </span><span class="m">0</span><span class="p">]]</span><span class="w">
</span></code></pre></div></div>
<p><strong>Example(s):</strong></p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kt">void</span> <span class="nf">bar</span><span class="o">()</span> <span class="o">{</span>
<span class="n">x</span><span class="o">.</span><span class="na">notify</span><span class="o">();</span>
<span class="c1">// If many threads are monitoring x, only one (and you won't know which) will be notified.</span>
<span class="c1">// use instead:</span>
<span class="n">x</span><span class="o">.</span><span class="na">notifyAll</span><span class="o">();</span>
<span class="o">}</span>
</code></pre></div></div>
<p><strong>Use this rule by referencing it:</strong></p>
<div class="language-xml highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nt">&lt;rule</span> <span class="na">ref=</span><span class="s">"category/java/multithreading.xml/UseNotifyAllInsteadOfNotify"</span> <span class="nt">/&gt;</span>
</code></pre></div></div>
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