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Reinier Zwitserloot
86 lines
4.8 KiB
HTML
86 lines
4.8 KiB
HTML
<!DOCTYPE html>
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<html><head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<link rel="stylesheet" type="text/css" href="../logi/reset.css" />
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<link rel="shortcut icon" href="../favicon.ico" type="image/x-icon" />
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<meta name="description" content="Spice up your java" />
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<title>@Synchronized</title>
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</head><body><div id="pepper">
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<div class="minimumHeight"></div>
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<div class="meat">
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<div class="header"><a href="../index.html">Project Lombok</a></div>
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<h1>@Synchronized</h1>
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<div class="byline"><code>synchronized</code> done right: Don't expose your locks.</div>
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<div class="overview">
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<h3>Overview</h3>
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<p>
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<code>@Synchronized</code> is a safer variant of the <code>synchronized</code> method modifier. Like <code>synchronized</code>, the
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annotation can be used on static and instance methods only. It operates similarly to the <code>synchronized</code> keyword, but it locks
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on different objects. The keyword locks on <code>this</code>, but the annotation locks on a field named <code>$lock</code>, which is private.<br />
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If the field does not exist, it is created for you. If you annotate a <code>static</code> method, the annotation locks on a static field
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named <code>$LOCK</code> instead.
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</p><p>
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If you want, you can create these locks yourself. The <code>$lock</code> and <code>$LOCK</code> fields will of course not be generated if you
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already created them yourself. You can also choose to lock on another field, by specifying it as parameter to the <code>@Synchronized</code>
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annotation. In this usage variant, the fields will not be created automatically, and you must explicitly create them yourself, or an error will be emitted.
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</p><p>
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Locking on <code>this</code> or your own class object can have unfortunate side-effects, as other code not under your control can lock on these
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objects as well, which can cause race conditions and other nasty threading-related bugs.
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</p>
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</div>
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<div class="snippets">
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<div class="pre">
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<h3>With Lombok</h3>
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<div class="snippet">@HTML_PRE@</div>
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</div>
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<div class="sep"></div>
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<div class="post">
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<h3>Vanilla Java</h3>
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<div class="snippet">@HTML_POST@</div>
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</div>
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</div>
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<div style="clear: left;"></div>
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<div class="overview confKeys">
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<h3>Supported configuration keys:</h3>
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<dl>
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<dt><code>lombok.synchronized.flagUsage</code> = [<code>warning</code> | <code>error</code>] (default: not set)</dt>
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<dd>Lombok will flag any usage of <code>@Synchronized</code> as a warning or error if configured.</dd>
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</dl>
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</div>
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<div class="overview">
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<h3>Small print</h3><div class="smallprint">
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<p>
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If <code>$lock</code> and/or <code>$LOCK</code> are auto-generated, the fields are initialized with an empty <code>Object[]</code> array, and not
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just a <code>new Object()</code> as most snippets showing this pattern in action use. Lombok does this because a new object is <em>NOT</em>
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serializable, but 0-size array is. Therefore, using <code>@Synchronized</code> will not prevent your object from being serialized.
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</p><p>
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Having at least one <code>@Synchronized</code> method in your class means there will be a lock field, but if you later remove all such methods,
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there will no longer be a lock field. That means your predetermined <code>serialVersionUID</code> changes. We suggest you <em>always</em> add
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a <code>serialVersionUID</code> to your classes if you intend to store them long-term via java's serialization mechanism. If you do so, removing
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all <code>@Synchronized</code> annotations from your method will not break serialization.
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</p><p>
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If you'd like to know why a field is not automatically generated when you choose your own name for the lock object: Because otherwise making a typo
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in the field name will result in a <em>very</em> hard to find bug!
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</p>
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</div>
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</div>
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<div class="footer">
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<a href="index.html">Back to features</a> | <a href="SneakyThrows.html">Previous feature (@SneakyThrows)</a> | <a href="GetterLazy.html">Next feature (@Getter(lazy=true))</a><br />
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<a href="../credits.html" class="creditsLink">credits</a> | <span class="copyright">Copyright © 2009-2015 The Project Lombok Authors, licensed under the <a href="http://www.opensource.org/licenses/mit-license.php">MIT license</a>.</span>
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