Synchronized Methods同步方法

The Java programming language provides two basic synchronization idioms: synchronized methods and synchronized statements. Java编程语言提供了两种基本的同步习惯用法：同步方法和同步语句。The more complex of the two, synchronized statements, are described in the next section. 下一节将描述这两个语句中更复杂的同步语句。This section is about synchronized methods.本节介绍同步方法。

To make a method synchronized, simply add the synchronized keyword to its declaration:要使方法同步，只需将synchronized关键字添加到其声明中：

public class SynchronizedCounter {
    private int c = 0;

    public synchronized void increment() {
        c++;
    }

    public synchronized void decrement() {
        c--;
    }

    public synchronized int value() {
        return c;
    }
}

If count is an instance of SynchronizedCounter, then making these methods synchronized has two effects:如果count是SynchronizedCounter的实例，则使这些方法同步有两个效果：

• First, it is not possible for two invocations of synchronized methods on the same object to interleave. 首先，同一对象上的两个同步方法调用不可能交错。When one thread is executing a synchronized method for an object, all other threads that invoke synchronized methods for the same object block (suspend execution) until the first thread is done with the object.当一个线程为一个对象执行同步方法时，调用同一对象块的同步方法的所有其他线程（暂停执行），直到第一个线程对该对象执行完毕。
• Second, when a synchronized method exits, it automatically establishes a happens-before relationship with any subsequent invocation of a synchronized method for the same object. 其次，当同步方法退出时，它会自动与同一对象的同步方法的任何后续调用建立“发生在之前”关系。This guarantees that changes to the state of the object are visible to all threads.这保证了对象状态的更改对所有线程都可见。

Note that constructors cannot be synchronized — using the synchronized keyword with a constructor is a syntax error. 请注意，构造函数不能同步—将synchronized关键字与构造函数一起使用是一个语法错误。Synchronizing constructors doesn't make sense, because only the thread that creates an object should have access to it while it is being constructed.同步构造函数没有意义，因为只有创建对象的线程才能在构造对象时访问它。

instances.add(this);

Synchronized methods enable a simple strategy for preventing thread interference and memory consistency errors: if an object is visible to more than one thread, all reads or writes to that object's variables are done through synchronized methods. 同步方法支持防止线程干扰和内存一致性错误的简单策略：如果一个对象对多个线程可见，则对该对象变量的所有读取或写入都通过synchronized方法完成。(An important exception: final fields, which cannot be modified after the object is constructed, can be safely read through non-synchronized methods, once the object is constructed) This strategy is effective, but can present problems with liveness, as we'll see later in this lesson.（一个重要的例外：构建对象后无法修改的final字段可以通过非同步方法安全地读取，一旦构建了对象）此策略是有效的，但可能会出现活性问题，我们将在本课后面看到。