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class="sidebar-link">Java Memory Management</a></li><li><a href="/java/java-performance-tuning.html" class="sidebar-link">Java Performance Tuning</a></li><li><a href="/java/benchmarks.html" class="sidebar-link">Benchmarks</a></li><li><a href="/java/fileupload-to-aws.html" class="sidebar-link">FileUpload to AWS</a></li><li><a href="/java/appdynamics-and-tibco-businessworks-instrumentation-for-easy-integration.html" class="sidebar-link">AppDynamics and TIBCO BusinessWorks Instrumentation for Easy Integration</a></li><li><a href="/java/the-classpath.html" class="sidebar-link">The Classpath</a></li><li><a href="/java/resources-on-classpath.html" class="sidebar-link">Resources (on classpath)</a></li><li><a href="/java/contributors.html" class="sidebar-link">The Contributors</a></li></ul></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><h1 id="concurrent-collections"><a href="#concurrent-collections" class="header-anchor">#</a> Concurrent Collections</h1> <h2 id="thread-safe-collections"><a href="#thread-safe-collections" class="header-anchor">#</a> Thread-safe Collections</h2> <p>By default, the various Collection types are not thread-safe.</p> <p>However, it's fairly easy to make a collection thread-safe.</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> threadSafeList <span class="token operator">=</span> <span class="token class-name">Collections</span><span class="token punctuation">.</span><span class="token function">synchronizedList</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Set</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> threadSafeSet <span class="token operator">=</span> <span class="token class-name">Collections</span><span class="token punctuation">.</span><span class="token function">synchronizedSet</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">HashSet</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Map</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">,</span> <span class="token class-name">String</span><span class="token punctuation">></span></span> threadSafeMap <span class="token operator">=</span> <span class="token class-name">Collections</span><span class="token punctuation">.</span><span class="token function">synchronizedMap</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">HashMap</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">,</span> <span class="token class-name">String</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><p>When you make a thread-safe collection, you should never access it through the original collection, only through the thread-safe wrapper.</p> <p>Starting in Java 5, <code>java.util.collections</code> has several new thread-safe collections that don't need the various <code>Collections.synchronized</code> methods.</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> threadSafeList <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">CopyOnWriteArrayList</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Set</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> threadSafeSet <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ConcurrentHashSet</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Map</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">,</span> <span class="token class-name">String</span><span class="token punctuation">></span></span> threadSafeMap <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ConcurrentHashMap</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">,</span> <span class="token class-name">String</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><h2 id="concurrent-collections-2"><a href="#concurrent-collections-2" class="header-anchor">#</a> Concurrent Collections</h2> <p>Concurrent collections are a generalization of thread-safe collections, that allow for a broader usage in a concurrent environment.</p> <p>While thread-safe collections have safe element addition or removal from multiple threads, they do not necessarily have safe iteration in the same context (one may not be able to safely iterate through the collection in one thread, while another one modifies it by adding/removing elements).</p> <p>This is where concurrent collections are used.</p> <p>As iteration is often the base implementation of several bulk methods in collections, like <code>addAll</code>, <code>removeAll</code>, or also collection copying (through a constructor, or other means), sorting, ... the use case for concurrent collections is actually pretty large.</p> <p>For example, the Java SE 5 <code>java.util.concurrent.CopyOnWriteArrayList</code> is a thread safe and concurrent <code>Lis</code>t implementation, its <a href="https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/CopyOnWriteArrayList.html" target="_blank" rel="noopener noreferrer">javadoc<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a> states :</p> <blockquote></blockquote> <p>The "snapshot" style iterator method uses a reference to the state of the array at the point that the iterator was created. This array never changes during the lifetime of the iterator, so interference is impossible and the iterator is guaranteed not to throw ConcurrentModificationException.</p> <p>Therefore, the following code is safe :</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">ThreadSafeAndConcurrent</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span> LIST <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">CopyOnWriteArrayList</span><span class="token generics"><span class="token punctuation"><</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">InterruptedException</span> <span class="token punctuation">{</span>
<span class="token class-name">Thread</span> modifier <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">ModifierRunnable</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Thread</span> iterator <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">IteratorRunnable</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
modifier<span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
iterator<span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
modifier<span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
iterator<span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">class</span> <span class="token class-name">ModifierRunnable</span> <span class="token keyword">implements</span> <span class="token class-name">Runnable</span> <span class="token punctuation">{</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator"><</span> <span class="token number">50000</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
LIST<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">Exception</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
e<span class="token punctuation">.</span><span class="token function">printStackTrace</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">class</span> <span class="token class-name">IteratorRunnable</span> <span class="token keyword">implements</span> <span class="token class-name">Runnable</span> <span class="token punctuation">{</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator"><</span> <span class="token number">10000</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">long</span> total <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token keyword">for</span><span class="token punctuation">(</span><span class="token class-name">Integer</span> inList <span class="token operator">:</span> LIST<span class="token punctuation">)</span> <span class="token punctuation">{</span>
total <span class="token operator">+=</span> inList<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>total<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">Exception</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
e<span class="token punctuation">.</span><span class="token function">printStackTrace</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>Another concurrent collection regarding iteration is <a href="https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ConcurrentLinkedQueue.html" target="_blank" rel="noopener noreferrer"><code>ConcurrentLinkedQueue</code><span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>, which states :</p> <blockquote></blockquote> <p>Iterators are weakly consistent, returning elements reflecting the state of the queue at some point at or since the creation of the iterator. They do not throw java.util.ConcurrentModificationException, and may proceed concurrently with other operations. Elements contained in the queue since the creation of the iterator will be returned exactly once.</p> <p>One should check the javadocs to see if a collection is concurrent, or not. The attributes of the iterator returned by the <code>iterator()</code> method ("fail fast", "weakly consistent", ...) is the most important attribute to look for.</p> <h3 id="thread-safe-but-non-concurrent-examples"><a href="#thread-safe-but-non-concurrent-examples" class="header-anchor">#</a> Thread safe but non concurrent examples</h3> <p>In the above code, changing the <code>LIST</code> declaration to</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span> LIST <span class="token operator">=</span> <span class="token class-name">Collections</span><span class="token punctuation">.</span><span class="token function">synchronizedList</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation"><</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><p>Could (and statistically will on most modern, multi CPU/core architectures) lead to exceptions.</p> <p>Synchronized collections from the <code>Collections</code> utility methods are thread safe for addition/removal of elements, but not iteration (unless the underlying collection being passed to it already is).</p> <h2 id="insertion-into-concurrenthashmap"><a href="#insertion-into-concurrenthashmap" class="header-anchor">#</a> Insertion into ConcurrentHashMap</h2> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">InsertIntoConcurrentHashMap</span>
<span class="token punctuation">{</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token class-name">ConcurrentHashMap</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Integer</span><span class="token punctuation">,</span> <span class="token class-name">SomeObject</span><span class="token punctuation">></span></span> concurrentHashMap <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ConcurrentHashMap</span><span class="token generics"><span class="token punctuation"><</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">SomeObject</span> value <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">SomeObject</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Integer</span> key <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token class-name">SomeObject</span> previousValue <span class="token operator">=</span> concurrentHashMap<span class="token punctuation">.</span><span class="token function">putIfAbsent</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> value<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>previousValue <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token comment">//Then some other value was mapped to key = 1. 'value' that was passed to</span>
<span class="token comment">//putIfAbsent method is NOT inserted, hence, any other thread which calls</span>
<span class="token comment">//concurrentHashMap.get(1) would NOT receive a reference to the 'value' </span>
<span class="token comment">//that your thread attempted to insert. Decide how you wish to handle </span>
<span class="token comment">//this situation.</span>
<span class="token punctuation">}</span>
<span class="token keyword">else</span>
<span class="token punctuation">{</span>
<span class="token comment">//'value' reference is mapped to key = 1.</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div></div> <footer class="page-edit"><div class="edit-link"><a href="https://github.com/devtut/generate/edit/master/docs/java/concurrent-collections.md" target="_blank" rel="noopener noreferrer">Edit this page on GitHub</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <!----></footer> <div class="page-nav"><p class="inner"><span class="prev">
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