Java多线程-3 线程同步之synchronized

1. synchronized解决的问题

If two or more threads share an object, and more than one thread updates variables in that shared object, race conditions may occur.



如果多个线程共享对象，且多个线程对共享对象的变量作修改时，竞争情况就可能发生了。

2. synchronized含义

来自stackoverflow的问题及回答：

Q:What does ‘synchronized’ mean?

A: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 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.

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.

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

Concurrency

Java Memory Model

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

什么意思呢？

官方说了:synchronized方法是为了防止线程之间互相干涉以及内存一致性错误：当一个对象对于多个线程可见时，所有对于这个对象变量的读和写必须通过synchronized 方法。

具体情况是：举个栗子–当你有2个线程读写同一个‘资源’，比如说一个名为 ‘foo’的变量， 你需要确保这些线程以一种原子操作访问该变量。如果没有用synchronized关键字，thread1可能看不到thread2对于变量’foo’所做的变化，更有甚者，thread1可能只看到了thread2对’foo’所作改变的一部分。这显然不是我们想要的结果！

少年，努力吧，等你达到了大佬“Brain Goetz”的高度，你就和并发合体了