“同步"是什么意思?

问题描述

我对 synchronized 关键字的用法和意义有一些疑问.

I have some questions regarding the usage and significance of the synchronized keyword.

• synchronized 关键字的意义是什么?
• 方法什么时候应该同步?
• 这在编程和逻辑上意味着什么?

• What is the significance of the synchronized keyword?
• When should methods be synchronized?
• What does it mean programmatically and logically?

推荐答案

synchronized 关键字是关于不同线程对相同变量、对象和资源的读取和写入.这在 Java 中不是一个微不足道的话题，但这里引用了 Sun:

The synchronized keyword is all about different threads reading and writing to the same variables, objects and resources. This is not a trivial topic in Java, but here is a quote from Sun:

synchronized 方法实现了一个简单的防止线程的策略干扰和内存一致性错误:如果对象可见多个线程，所有读取或写入该对象的变量是通过同步方法完成.

简而言之:当您有两个线程正在读取和写入同一个资源"时，比如说一个名为 foo 的变量，您需要以确保这些线程以原子方式访问变量.如果没有 synchronized 关键字，您的线程 1 可能看不到线程 2 对 foo 所做的更改，或者更糟的是，它可能只更改了一半.这不会是您逻辑上所期望的.

In a very, very small nutshell: When you have two threads that are reading and writing to the same 'resource', say a variable named foo, you need to ensure that these threads access the variable in an atomic way. Without the synchronized keyword, your thread 1 may not see the change thread 2 made to foo, or worse, it may only be half changed. This would not be what you logically expect.

同样，这是 Java 中的一个重要话题.要了解更多信息，请在 SO 和 Internet 上探索以下主题:

Again, this is a non-trivial topic in Java. To learn more, explore topics here on SO and the Interwebs about:

• 并发
• Java 内存模型

继续探索这些主题，直到名称 Brian Goetz" 在您的大脑中与术语并发性" 永久关联.

Keep exploring these topics until the name "Brian Goetz" becomes permanently associated with the term "concurrency" in your brain.

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