学习Java，使用synchronized关键字 [英] Learning Java, use of synchronized keyword

问题描述

因此我使用 synchronized 关键字进行测试。这是我尝试的一个例子：

so i was testing with synchronized keyword. Here is an example that I tried:

public class MyTest {
    static int i = 0;
    public static void main(String[] args) {
        new Thread(t1).start();
        new Thread(t2).start();
    }

    private static void countMe(String name){
        i++;
        System.out.println("Current Counter is: " + i + ", updated by: " + name);
    }

    private static Runnable t1 = new Runnable() {
        public void run() {
            try{
                for(int i=0; i<5; i++){
                    countMe("t1");
                }
            } catch (Exception e){}

        }
    };

    private static Runnable t2 = new Runnable() {
        public void run() {
            try{
                for(int i=0; i<5; i++){
                    countMe("t2");
                }
            } catch (Exception e){}
       }
    };
}  

当我运行它时，调用 countMe的输出（）来自两个线程的方法生成此输出：

When I run it, the output of calling countMe() method from two threads generates this output:

Current Counter is: 1
Current Counter is: 2
Current Counter is: 4
Current Counter is: 5
Current Counter is: 6
Current Counter is: 7
Current Counter is: 3
Current Counter is: 8
Current Counter is: 9
Current Counter is: 10  

当我将方法 countMe（）更改为：

private synchronized static void countMe(){
        i++;
        System.out.println("Current Counter is: " + i);
}  

我得到这个输出：

Current Counter is: 1
Current Counter is: 2
Current Counter is: 3
Current Counter is: 4
Current Counter is: 5
Current Counter is: 6
Current Counter is: 7
Current Counter is: 8
Current Counter is: 9
Current Counter is: 10  

虽然这让我清楚地了解了 synchronized ，我想知道还有其他原因，我们可以使用 synchronized 。或者我在这里做了什么，是我们需要使用这个 synchronized 关键字的唯一原因？

Although this gives me clear understanding the purpose of synchronized, I want to know is there any other reason as well, that we can use synchronized. Or what I have done here, is the only eason why we need the use of this synchronized keyword?

谢谢。

编辑：我感到困惑的另一件事是，在第一次输出时为什么计数器在7之后变为3.这似乎有点不可能对我而言，每次尝试都会发生类似的结果，这是正常的吗？

Another thing that I am confused with is that in first output why the counter went to 3 after 7. It seems a bit impossible to me, but similar results do happen every time I try, is this normal?

推荐答案

两件事：

首先，对同一对象的两次同步方法的调用不可能进行交错。当一个线程正在为一个对象执行一个synchronized方法时，所有其他线程都会调用同一个对象的同步方法阻塞（暂停执行），直到第一个线程完成该对象为止。

First, it is not possible for two invocations of synchronized methods on the same object to interleave. 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.

其次，当synchronized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before-before关系。这可以保证对所有线程都可以看到对象状态的更改。

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. This guarantees that changes to the state of the object are visible to all threads.

同步方法启用了一个简单的策略来防止线程干扰和内存一致性错误：如果一个对象是对多个线程可见，对该对象变量的所有读取或写入都是通过同步方法完成的。 （一个重要的例外：构造对象后无法修改的final字段，一旦构造了对象，就可以通过非同步方法安全地读取）。

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. (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).

来源： http://docs.oracle.com/javase/tutorial/essential /concurrency/syncmeth.html

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