先上代码：
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public class testthread {
private static final logger logger = loggerfactory.getlogger(testthread.class);
private final class myrunnable implements runnable {
private int i;
public myrunnable() {
this.i = 10;
}
public void run() {
while(i > 0) {
synchronized (this) {
if (i > 0) {
i--;
logger.debug("{} buy one ticket, {} left. ", thread.currentthread().getname(), i);
}
}
}
}
}
@test
public void testrunable() throws interruptedexception{
myrunnable myrunnable = new myrunnable();
thread th1 = new thread(myrunnable);
thread th2 = new thread(myrunnable);
th1.start();
th2.start();
th1.join();
th2.join();
}
}
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楼上的代码很简单，模拟一个售票系统。通过两个thread对象开启两条线程同时运行一个myrunnable实例。
几个注意点：
1. 没有加上synchronised关键词的话，即
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public void run() {
while(i > 0) {
if (i > 0) {
i--;
logger.debug("{} buy one ticket, {} left. ", thread.currentthread().getname(), i);
}
}
}
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系统的运行结果：
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thread-1 buy one ticket, 8 left.
thread-2 buy one ticket, 8 left.
thread-2 buy one ticket, 6 left.
thread-1 buy one ticket, 6 left.
thread-2 buy one ticket, 5 left.
thread-1 buy one ticket, 4 left.
thread-2 buy one ticket, 3 left.
thread-1 buy one ticket, 2 left.
thread-2 buy one ticket, 1 left.
thread-1 buy one ticket, 0 left.
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可以看到，缺少同步的程序输出明显有问题。
2. 在进入同步代码块之后，还需要对i的值再进行一次判断，即，如果不加if判断：
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public void run() {
while(i > 0) {
synchronized (this) {
i--;
logger.debug("{} buy one ticket, {} left. ", thread.currentthread().getname(), i);
}
}
}
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程序的运行结果为：
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thread-2 buy one ticket, 9 left.
thread-2 buy one ticket, 8 left.
thread-2 buy one ticket, 7 left.
thread-2 buy one ticket, 6 left.
thread-2 buy one ticket, 5 left.
thread-2 buy one ticket, 4 left.
thread-2 buy one ticket, 3 left.
thread-2 buy one ticket, 2 left.
thread-2 buy one ticket, 1 left.
thread-2 buy one ticket, 0 left.
thread-1 buy one ticket, -1 left.
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可以看出，出现了“多卖”的现象， 所以需要在进入同步代码块中再进行一次if判断。
总结
synchronised用于互斥访问共享变量， 并在同步代码块中使用if判断更新共享变量。
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持快网idc。
原文链接：https://blog.csdn.net/NewUser2771894007/article/details/79630721
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