从头认识多线程-2.1 局部变量与实例变量的线程安全

这一章节我们来讨论一下局部变量与实例变量的线程安全。

1.结论

局部变量线程安全的

实例变量不是线程安全的

2.代码清单

package com.ray.deepintothread.ch02.topic_1;

public class ThreadSafeOfLocalVariable {
	public static void main(String[] args) throws InterruptedException {
		ThreadOne threadOne = new ThreadOne();
		Thread thread = new Thread(threadOne);
		Thread thread1 = new Thread(threadOne);
		thread.start();
		thread1.start();
	}
}

class ThreadOne implements Runnable {

	@Override
	public void run() {
		int count = 0;
		for (int i = 0; i < 5; i++) {
			System.out.println(count++);
			try {
				Thread.sleep(100);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}
}

输出：

0
0
1
1
2
2
3
3
4
4

从输出可以看出，count在++的时候没有被另一个线程篡改，因此，局部变量是线程安全的

package com.ray.deepintothread.ch02.topic_1;

public class ThreadSafeOfInstanceVariable {
	public static void main(String[] args) throws InterruptedException {
		ThreadFour threadFour = new ThreadFour();
		Thread thread = new Thread(threadFour);
		Thread thread1 = new Thread(threadFour);
		thread.start();
		thread1.start();
	}
}

class ThreadFour implements Runnable {
	private int count = 0;

	@Override
	public void run() {
		for (int i = 0; i < 5; i++) {
			System.out.println(count++);
			try {
				Thread.sleep(100);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}
}

输出：

0
0
1
2
3
4
5
6
7
8

我们来观察输出结果，可以看到从第四个结果开始，其实两个线程之间就开始互相篡改，而且输出结果看上去是无序的，因此输出结果跟上面的完全不一样

但是，当我们在run上面加上同步标识符，他的输出结果将会变成有序的

package com.ray.deepintothread.ch02.topic_1;

public class ThreadSafeOfInstanceVariable {
	public static void main(String[] args) throws InterruptedException {
		ThreadTow threadTow = new ThreadTow();
		Thread thread = new Thread(threadTow);
		Thread thread1 = new Thread(threadTow);
		thread.start();
		thread1.start();
	}
}

class ThreadTow implements Runnable {
	private int count = 0;

	private synchronized void test() {
		for (int i = 0; i < 5; i++) {
			System.out.println(count++);
			try {
				Thread.sleep(100);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}

	@Override
	public void run() {
		test();
	}
}

输出：

0
1
2
3
4
5
6
7
8
9

总结：这一章节主要讲述了局部变量与实例变量的线程安全。

这一章节就到这里，谢谢

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我的github：https://github.com/raylee2015/DeepIntoThread

目录：http://blog.csdn.net/raylee2007/article/details/51204573