块级同步 [英] Block level synchronization

问题描述

传递给synchronized的参数有什么意义?

What is the significance of parameter passed to synchronized?

synchronized ( parameter )
{

}

实现块级同步.在某处我看到像

to achieve block level synchronization. Somewhere i saw code like

class test
{
    public static final int lock =1;

    ...

    synchronized(lock){
       ...
    }
}

我不明白这段代码的目的.

I don't understand the purpose of this code.

谁能给我一个更好的例子和/或解释一下?

Can anyone give me a better example and/or explain it?

推荐答案

锁定参考.基本上两个线程不会同时执行使用相同引用同步的代码块.正如 Cletus 所说，同步 方法 大体上等同于在方法内部使用 synchronized (this).

It's the reference to lock on. Basically two threads won't execute blocks of code synchronized using the same reference at the same time. As Cletus says, a synchronized method is mostly equivalent to using synchronized (this) inside the method.

我非常希望您看到的示例代码不是完全那样 - 您正在尝试对原始变量进行同步.同步仅适用于监视器(通过引用)——即使它 是合法的代码，x 也会被装箱，这会导致一些非常 奇怪的行为，因为一些整数总是被装箱到相同的引用，而其他的每次装箱时都会创建一个新对象.幸运的是，Java 编译器意识到这是一个非常糟糕的主意，并且会为您发布的代码提供编译时错误.

I very much hope that the example code you saw wasn't quite like that - you're trying to synchronize on a primitive variable. Synchronization only works on a monitor (via a reference) - even if it were legal code, x would be boxed, which would lead to some very odd behaviour, as some integers will always be boxed to the same references, and others will create a new object each time you box. Fortunately, the Java compiler realises this is a very bad idea, and will give you a compile-time error for the code you've posted.

更合理的代码是:

class Test
{
    private static final Object lock = new Object();

    ...

    synchronized(lock){
       ...
    }
}

我已将锁设为私有，并将其类型更改为 Object.它是否应该是静态的取决于具体情况——如果您想从多个线程访问/更改静态数据，通常会使用静态变量；当您想要从多个线程访问/更改每个实例的数据时，实例变量通常用于锁定.

I've made the lock private, and changes its type to Object. Whether or not it should be static depends on the situation - basically a static variable is usually used if you want to access/change static data from multiple threads; instance variables are usually used for locks when you want to access/change per-instance data from multiple threads.

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