关于线程安全

线程安全的本质体现在两个方面:

Ａ变量安全：多线程同时运行一段代码

Ｂ线程同步：一个线程还没执行完，另一个线程又进来接着执行。

看个简单的例子。

public class ThreadSafe implements java.lang.Runnable {
	
	int num = 1;
	public void run() {
		for (int i = 0; i < 3; i++) {
			num = num + 1;
			try {
				Thread.sleep(2000);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			System.out.println("num is value +==="+Thread.currentThread().getName()+"---------" + num);
		}
	}

}

package com.java.thread.test;

public class TestMan {
public static void main(String[] args) {
Runnable safe=new ThreadSafe();
Thread thread1=new Thread(safe,"thread1");
Thread thread2=new Thread(safe,"thread2");
thread1.start();
thread2.start();
}

}

运行结果

num is value +===thread2---------3
num is value +===thread1---------4
num is value +===thread2---------5
num is value +===thread1---------6
num is value +===thread1---------7
num is value +===thread2---------7

很明显是错误的，应为两个线程共享同一个变量。这里就是变量的安全问题。

解决办法：

１抛弃单实例，多线程的方式，用多实例，多线程的方式，这样就和单线程是一个样了，不会出错，但是是最接近传统的编程模式

２不要用类的实例变量，经可能把变量封装到方法内部。

１类的解决办法的代码。

public class TestMan {
	public static void main(String[] args) {
		Runnable safe=new ThreadSafe();
		Runnable safe2=new ThreadSafe();
		Thread thread1=new Thread(safe,"thread1");
		Thread thread2=new Thread(safe2,"thread2");
		thread1.start();
		thread2.start();
	}

}

运行结果

num is value +===thread1---------2
num is value +===thread2---------2
num is value +===thread1---------3
num is value +===thread2---------3
num is value +===thread1---------4
num is value +===thread2---------4
２类解决办法的代码

public class ThreadSafe implements java.lang.Runnable {
	
	public void run() {
		int num = 1; // 我在内部
		for (int i = 0; i < 3; i++) {
			num = num + 1;
			try {
				Thread.sleep(2000);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			System.out.println("num is value +==="+Thread.currentThread().getName()+"---------" + num);
		}
	}

}

public class TestMan {
	public static void main(String[] args) {
		Runnable safe=new ThreadSafe();

		Thread thread1=new Thread(safe,"thread1");
		Thread thread2=new Thread(safe,"thread2");
		thread1.start();
		thread2.start();
	}

}

运行结果

num is value +===thread2---------2
num is value +===thread1---------2
num is value +===thread1---------3
num is value +===thread2---------3
num is value +===thread1---------4
num is value +===thread2---------4

这两种办法，比较推荐适用第二个办法，就是把变量经可能的封装到风发内部，这样他们就是线程的私有变量了。另外，从jdk1.2后，推出了threadlocal　对象，它作为线程的一个局部变量，可以为每个线程创建一个副本，用来保存每个线程的属性，各是各的，互不干扰。单每个threadlocal变量只能保存一个变量，假如有多个变量要保存，那么就要写多个threadlocal对象。

我们把代码改写一下。

public class ThreadSafe implements java.lang.Runnable {
	ThreadLocal<Integer> local=new ThreadLocal<Integer>();
	public void run() {
		for (int i = 0; i < 3; i++) {
			if(local.get()==null){
				local.set(new Integer(1));
			}
			int num=local.get().intValue();
			num=num+1;
			local.set(new Integer(num));
			try {
				Thread.sleep(2000);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			System.out.println("num is value +==="+Thread.currentThread().getName()+"---------" + local.get().intValue());
		}
	}

}

public class TestMan {
	public static void main(String[] args) {
		Runnable safe=new ThreadSafe();
		Thread thread1=new Thread(safe,"thread1");
		Thread thread2=new Thread(safe,"thread2");
		thread1.start();
		thread2.start();
	}

}

运行结果

num is value +===thread2---------2
num is value +===thread1---------2
num is value +===thread1---------3
num is value +===thread2---------3
num is value +===thread1---------4
num is value +===thread2---------4

结果是一样的，所以这里变量安全有３个办法可以解决。

然后在说说线程的同步的问题。我们看上面的运行结果。

num is value +===thread2---------2
num is value +===thread1---------2
num is value +===thread1---------3
num is value +===thread2---------3
num is value +===thread1---------4
num is value +===thread2---------4

就　可以看出他们不是线程同步的，是thread1和thread2在交替执行的。在有些情况下，要求一段代码在运行的过程中是一个不可分割的实体，就是原子的。就是说当已经有线程在执行这段代码的时候，其他的线程必须等待他执行完毕后才能竟来执行，这就是所谓的线程同步。

java通过同步锁来执行线程的同步和等待，也就是说，要不间断执行的代码需要放在synchronized关键字标识的代码块中。可以用来修饰代码块，也可以修饰方法。

public class ThreadSafe implements java.lang.Runnable{
	public synchronized void run() {
		int num = 1;
		for (int i = 0; i < 3; i++) {
			num = num + 1;
			try {
				Thread.sleep(2000);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			System.out.println("num is value +==="+Thread.currentThread().getName()+"---------" + num);
		}
	}


}

public class TestMan {
	public static void main(String[] args) {
		Runnable safe=new ThreadSafe();
		Thread thread1=new Thread(safe,"thread1");
		Thread thread2=new Thread(safe,"thread2");
		thread1.start();
		thread2.start();
	}

}

运行结果

um is value +===thread1---------2
num is value +===thread1---------3
num is value +===thread1---------4
num is value +===thread2---------2
num is value +===thread2---------3
num is value +===thread2---------4

可以看到thread1运行结束后thread2才开始运行的。代码还可以这么写

public class ThreadSafe implements java.lang.Runnable {
	public void run() {
		int num = 1;
		synchronized (this) {
			for (int i = 0; i < 3; i++) {
				num = num + 1;
				try {
					Thread.sleep(2000);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				System.out.println("num is value +==="
						+ Thread.currentThread().getName() + "---------" + num);
			}
		}
	}

}

在启用同步锁机制以后，需要避免

１无线等待，，线程Ｂ等待线程Ａ执行完毕，然后线程Ａ确进入了死循环。

２循环等待：两个线程相互调用，都要求要同步执行，这个时候就先会循环等待，我等你执行，你也在等我执行，这个时候就死锁了