blocks|key|3347252|text|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3347253|由于静态方法没有关联的对象，所以synchronized关键字会锁定类而不是对象吗？|blockquote|3347254|3347255|3347256|是。:)|3347257|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|-4|4|5|6|J|7|@]|8|@]|9|$]]|$1|A|3|B|4|C|6|K|7|@]|8|@]|9|$]]|$1|D|3|-4|4|5|6|L|7|@]|8|@]|9|$]]|$1|E|3|-4|4|5|6|M|7|@]|8|@]|9|$]]|$1|F|3|G|4|5|6|N|7|@]|8|@]|9|$]]|$1|H|3|-4|4|5|6|O|7|@]|8|@]|9|$]]]|I|$]]

<blockquote>
 Since a static method has no associated object, will the synchronized keyword lock on the class, instead of the object?
</blockquote>

Yes. :)

blocks|key|3134387|text|只是为了给奥斯卡增加一点细节(简洁得令人愉悦！)回答，Java+Language+Specification的相关部分是8.4.3.6,+'synchronized+Methods'|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|3134388|3134389|同步方法在执行之前获取一个监视器(§17.1)。对于类(静态)方法，使用与该方法的类的class对象相关联的监视器。对于一个实例方法，使用与此关联的监视器(调用该方法的对象)。|blockquote|3134390|3134391|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.oracle.com/javase/specs/jls/se7/html/jls-8.html#jls-8.4.3.6|1|https://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.1^0|1O|V|0|0|0|H|5|1|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@$A|T|B|U|1|V]]|C|$]]|$1|D|3|-4|5|6|7|W|8|@]|9|@]|C|$]]|$1|E|3|F|5|G|7|X|8|@]|9|@$A|Y|B|Z|1|10]]|C|$]]|$1|H|3|-4|5|6|7|11|8|@]|9|@]|C|$]]|$1|I|3|-4|5|6|7|12|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]|Q|$5|L|M|N|C|$O|R]]]]

Just to add a little detail to Oscar's (pleasingly succinct!) answer, the relevant section on the Java Language Specification is <a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-8.html#jls-8.4.3.6" rel="nofollow noreferrer">8.4.3.6, 'synchronized Methods'</a>:

<blockquote>
 A synchronized method acquires a monitor (<a href="https://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.1" rel="nofollow noreferrer">§17.1</a>) before it executes. For a class (static) method, the monitor associated with the Class object for the method's class is used. For an instance method, the monitor associated with this (the object for which the method was invoked) is used. 
</blockquote>

blocks|key|352162|text|你必须注意的一点(几个程序员通常会落入这个陷阱)是同步的静态方法和同步的非静态方法之间没有联系，即：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|352163|class+A+{
++++static+synchronized+f()+{...}
++++synchronized+g()+{...}
}|code-block|syntax|javascript|352164|Main：|352165|A+a+=+new+A();|352166|线程1：|352167|A.f();|352168|线程2：|352169|a.g();|352170|f()和g()彼此不同步，因此可以完全并发执行。|352171|entityMap^0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Y|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|10|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|11|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|12|8|@]|9|@]|A|$]]|$1|Q|3|R|5|D|7|13|8|@]|9|@]|A|$E|F]]|$1|S|3|T|5|6|7|14|8|@]|9|@]|A|$]]|$1|U|3|-4|5|6|7|15|8|@]|9|@]|A|$]]]|V|$]]

One point you have to be careful about (several programmers generally fall in that trap) is that there is no link between synchronized static methods and sync'ed non static methods, ie:

<pre><code>class A {
 static synchronized f() {...}
 synchronized g() {...}
}
</code></pre>

Main:

<pre><code>A a = new A();
</code></pre>

Thread 1:

<pre><code>A.f();
</code></pre>

Thread 2:

<pre><code>a.g();
</code></pre>

f() and g() are not synchronized with each other and thus can execute totally concurrently.

blocks|key|3347353|text|除非您按如下方式实现g()：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3347354|g()+{
++++synchronized(getClass())+{
++++++++...
++++}
}|code-block|syntax|javascript|3347355|当我想要实现对象的不同实例之间的互斥时(例如，在访问外部资源时需要)，我发现这个模式也很有用。|3347356|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Unless you implement g() as follows:

<pre><code>g() {
 synchronized(getClass()) {
 ...
 }
}
</code></pre>

I find this pattern useful also when I want to implement mutual exclusion between different instances of the object (which is needed when accesing an external resource, for example).

blocks|key|3347477|text|请查看Intrinsic+Locks+and+Synchronization上的oracle文档页面|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|3347478|3347479|您可能想知道调用静态同步方法时会发生什么，因为静态方法与类关联，而不是与对象关联。在这种情况下，线程获取与类关联的类对象的内部锁。因此，对类的静态字段的访问由一个锁控制，该锁不同于类的任何实例的锁。|blockquote|style|BOLD|3347480|3347481|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.oracle.com/javase/tutorial/essential/concurrency/locksync.html^0|3|Z|0|0|0|15|D|1T|Q|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@$A|T|B|U|1|V]]|C|$]]|$1|D|3|-4|5|6|7|W|8|@]|9|@]|C|$]]|$1|E|3|F|5|G|7|X|8|@$A|Y|B|Z|H|I]|$A|10|B|11|H|I]]|9|@]|C|$]]|$1|J|3|-4|5|6|7|12|8|@]|9|@]|C|$]]|$1|K|3|-4|5|6|7|13|8|@]|9|@]|C|$]]]|L|$M|$5|N|O|P|C|$Q|R]]]]

Have a look at oracle documentation page on <a href="https://docs.oracle.com/javase/tutorial/essential/concurrency/locksync.html" rel="nofollow">Intrinsic Locks and Synchronization</a>

<blockquote>
 You might wonder what happens when a static synchronized method is invoked, since a static method is associated with a class, not an object. In this case, the thread acquires the intrinsic lock for the Class object associated with the class. Thus access to class's static fields is controlled by a lock that's distinct from the lock for any instance of the class.
</blockquote>

blocks|key|352287|text|静态方法也有一个关联的对象。它属于JDK工具包中的Class.class文件。当模板文件加载到ram中时，Class.class会创建它的一个实例，称为.class对象。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|352288|例如:-当您尝试从现有的customer类创建对象时，如|352289|Customer+c+=+new+Customer();|code-block|syntax|javascript|352290|将Customer.class加载到内存中。在那一刻，JDK工具包中的Class.class创建了一个名为Template+Object的对象，并将该Customer.class加载到该模板中，该Customer.class的object.Static成员成为该模板对象中的属性和方法。|352291|因此静态方法或属性也有一个对象|352292|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|Q|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

A static method also has an associated object. It belongs to Class.class file in JDK toolkit. When the .class file load into the ram, the Class.class creates a instance of it called template object.

Eg :- when you try to create object from existing customer class like

<pre><code>Customer c = new Customer();
</code></pre>

The Customer.class load into RAM. In that moment Class.class in JDK toolkit creates an Object called Template object and load that Customer.class into that template object.Static members of that Customer.class become attributes and methods in that template object.

So a static method or attribute also has an object

blocks|key|3134770|text|下面的例子更清楚地说明了类和对象锁之间的关系，希望下面的例子也能对其他人有所帮助。)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3134771|例如，我们有以下方法，一个是acquire类，另一个是acquire对象锁：|3134772|public+class+MultiThread+{

++++public+static+synchronized+void+staticLock()+throws+InterruptedException+{
++++++++for+(int+i+=+0;+i+<+10;+i%2B%2B)+{
++++++++++++Thread.sleep(100);
++++++++++++System.out.println(Thread.currentThread().getName()+%2B+"+"+%2B+i);
++++++++}
++++}

++++public+synchronized+void+objLock()+throws+InterruptedException+{
++++++++for+(int+i+=+0;+i+<+10;+i%2B%2B)+{
++++++++++++Thread.sleep(100);
++++++++++++System.out.println(Thread.currentThread().getName()+%2B+"+"+%2B+i);
++++++++}
++++}
}|code-block|syntax|javascript|3134773|因此，现在我们可以有以下场景：|3134774|3134775|当使用same+Object的线程同时尝试访问objLock+或+staticLock方法时(即两个线程都试图访问相同的方法)|ordered-list-item|offset|length|style|BOLD|CODE|3134776|3134777|Thread-0+0+Thread-0+1+Thread-0+2+Thread-0+3+Thread-0+4+Thread-1+0+Thread-1+1+Thread-1+2+Thread-1+3+Thread-1+4|3134778|当使用相同对象的线程尝试同时访问staticLock和objLock方法时(尝试访问不同的方法)|unordered-list-item|3134779|3134780|Thread-0+0+Thread-1+0+Thread-0+1+Thread-1+1+Thread-0+2+Thread-1+2+Thread-1+3+Thread-0+3+Thread-0+4+Thread-1+4|3134781|当使用不同对象的线程尝试访问staticLock方法时|3134782|3134783|Thread-0+0+Thread-0+1+Thread-0+2+Thread-0+3+Thread-0+4+Thread-1+0+Thread-1+1+Thread-1+2+Thread-1+3+Thread-1+4|3134784|当使用不同对象的线程尝试访问objLock方法时|3134785|3134786|线程-0+0线程-1+0线程-0+1线程-1+1线程-0+2线程-1+2线程-1+3线程-0+3线程-0+4线程-1+4|3134787|entityMap^0|0|0|0|0|0|3|B|V|1|N|7|X|A|0|0|0|3|4|G|A|R|7|0|0|0|3|4|E|A|0|0|0|3|4|E|7|0|0|0^^$0|@$1|2|3|4|5|6|7|1E|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|1F|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|1G|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|1H|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|1I|8|@]|9|@]|A|$]]|$1|L|3|M|5|N|7|1J|8|@$O|1K|P|1L|Q|R]|$O|1M|P|1N|Q|R]|$O|1O|P|1P|Q|S]|$O|1Q|P|1R|Q|S]]|9|@]|A|$]]|$1|T|3|-4|5|6|7|1S|8|@]|9|@]|A|$]]|$1|U|3|V|5|6|7|1T|8|@]|9|@]|A|$]]|$1|W|3|X|5|Y|7|1U|8|@$O|1V|P|1W|Q|R]|$O|1X|P|1Y|Q|S]|$O|1Z|P|20|Q|S]]|9|@]|A|$]]|$1|Z|3|-4|5|6|7|21|8|@]|9|@]|A|$]]|$1|10|3|11|5|6|7|22|8|@]|9|@]|A|$]]|$1|12|3|13|5|Y|7|23|8|@$O|24|P|25|Q|R]|$O|26|P|27|Q|S]]|9|@]|A|$]]|$1|14|3|-4|5|6|7|28|8|@]|9|@]|A|$]]|$1|15|3|16|5|6|7|29|8|@]|9|@]|A|$]]|$1|17|3|18|5|Y|7|2A|8|@$O|2B|P|2C|Q|R]|$O|2D|P|2E|Q|S]]|9|@]|A|$]]|$1|19|3|-4|5|6|7|2F|8|@]|9|@]|A|$]]|$1|1A|3|1B|5|6|7|2G|8|@]|9|@]|A|$]]|$1|1C|3|-4|5|6|7|2H|8|@]|9|@]|A|$]]]|1D|$]]

Below examples gives more clarity between class and object lock, hope below example will help others as well :)

For example we have below methods one acquire class and other acquire object lock :

<pre><code>public class MultiThread {

 public static synchronized void staticLock() throws InterruptedException {
 for (int i = 0; i &lt; 10; i++) {
 Thread.sleep(100);
 System.out.println(Thread.currentThread().getName() + " " + i);
 }
 }

 public synchronized void objLock() throws InterruptedException {
 for (int i = 0; i &lt; 10; i++) {
 Thread.sleep(100);
 System.out.println(Thread.currentThread().getName() + " " + i);
 }
 }
}
</code></pre>

So, now we can have following scenarios :

<ol>
<li>When threads using same Object tries to access <code>objLock</code> OR <code>staticLock</code> method same time (i.e. both threads are trying to access same method)

<pre><code>Thread-0 0
Thread-0 1
Thread-0 2
Thread-0 3
Thread-0 4
Thread-1 0
Thread-1 1
Thread-1 2
Thread-1 3
Thread-1 4
</code></pre></li>
<li>When threads using same Object tries to access <code>staticLock</code> and <code>objLock</code> methods same time (tries accessing different methods)

<pre><code>Thread-0 0
Thread-1 0
Thread-0 1
Thread-1 1
Thread-0 2
Thread-1 2
Thread-1 3
Thread-0 3
Thread-0 4
Thread-1 4
</code></pre></li>
<li>When threads using different Object tries to access <code>staticLock</code> method

<pre><code>Thread-0 0
Thread-0 1
Thread-0 2
Thread-0 3
Thread-0 4
Thread-1 0
Thread-1 1
Thread-1 2
Thread-1 3
Thread-1 4
</code></pre></li>
<li>When threads using different Object tries to access <code>objLock</code> method

<pre><code>Thread-0 0
Thread-1 0
Thread-0 1
Thread-1 1
Thread-0 2
Thread-1 2
Thread-1 3
Thread-0 3
Thread-0 4
Thread-1 4
</code></pre></li>
</ol>

blocks|key|3347540|text|对于那些不熟悉的人来说，静态同步方法锁定在类对象上，例如对于string类，它的String.class，而实例同步方法锁定在对象的当前实例上，用Java语言中的“this”关键字表示。由于这两个对象不同，它们具有不同锁，因此当一个线程执行静态同步方法时，java中的其他线程不需要等待线程返回，而是将获取表示为字节.class文字的单独锁并进入静态同步方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3347541|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

For those who are not familiar static synchronized method locked on class object e.g. for string class its String.class while instance synchronized method locks on current instance of Object denoted by “this” keyword in Java. Since both of these object are different they have different lock so while one thread is executing static synchronized method , other thread in java doesn’t need to wait for that thread to return instead it will acquire separate lock denoted byte .class literal and enter into static synchronized method.

The Java documentation says: 

<blockquote>
 It is not possible for two invocations of synchronized methods on the
 same object to interleave.
</blockquote>

What does this mean for a static method? Since a static method has no associated object, will the synchronized keyword lock on the class, instead of the object?

Java synchronized static methods: lock on object or class

Java

Java文档中写道：对同一对象的两次同步方法调用不可能交错。这对静态方法意味着什么？由于静态方法没有关联的对象，那么synchronized关键字会锁定类而不是对象吗？

问Java同步静态方法:锁定对象或类
EN

回答 8

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