Java 线程学习总结

isAlive(),join()的使用
isAlive()方法在Thread中定义:final bollean isAlive() ,
所以只能在Thread类的实例或其子类中调用.
一个更经常使用的方法是调用join()方法来等待另一个线程的结束.它的定义如下:
final void join() throws InterruptedException
这个方法一直等待,直到它调用的线程终止.

package mythread;

class NewThread implements Runnable {
	String name; // name of thread

	Thread t;

	NewThread(String threadname) {
		name = threadname;
		t = new Thread(this, name);
		System.out.println("New thread: " + t);
		t.start(); // Start the thread
	}

	// This is the entry point for thread.
	public void run() {
		try {
			for (int i = 5; i > 0; i--) {
				System.out.println(name + ": " + i);
				Thread.sleep(1000);
			}
		} catch (InterruptedException e) {
			System.out.println(name + " interrupted.");
		}
		System.out.println(name + " exiting.");
	}
}




public class DemoJoinIsAlive {
	public static void main(String args[]) {
		NewThread ob1 = new NewThread("One");
		NewThread ob2 = new NewThread("Two");
		NewThread ob3 = new NewThread("Three");

		System.out.println("Thread One is alive: " + ob1.t.isAlive());
		System.out.println("Thread Two is alive: " + ob2.t.isAlive());
		System.out.println("Thread Three is alive: " + ob3.t.isAlive());
		// wait for threads to finish
		try {
			System.out.println("Waiting for threads to finish.");
			ob1.t.join();
			ob2.t.join();
			ob3.t.join();
		} catch (InterruptedException e) {
			System.out.println("Main thread Interrupted");
		}

		System.out.println("Thread One is alive: " + ob1.t.isAlive());
		System.out.println("Thread Two is alive: " + ob2.t.isAlive());
		System.out.println("Thread Three is alive: " + ob3.t.isAlive());

		System.out.println("Main thread exiting.");
	}
}

同步
可以用两种方式实现:使用同步方法;同步语句(同步代码块).它们都使用关键字synchronized.
一.使用同步方法:
直接上代码:

/**
 * 演示同步方法的使用!!!
 * NOTES:
 * java 中每一个对象都有一个隐含锁(或称监控器),
 * 记住,一旦一个线程进入一个实例的任何同步方法,别的线程将不能进入同一实例的其他同步方法,
 * 因为它没有持有此实例的(隐含)锁,但是该实例的非同步方法仍然能够被调用.
 * 
 */

class Callme {
//比较下面两行方法的声明,当加synchronized时同步,否则不会同步.	
	//	synchronized void call(String msg) {
		void call(String msg) {
		System.out.print("[" + msg);
		try {
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			System.out.println("Interrupted");
		}
		System.out.println("]");
	}
}

class Caller implements Runnable {
	String msg;

	Callme target;

	Thread t;

	public Caller(Callme targ, String s) {
		target = targ;
		msg = s;
		t = new Thread(this);
		t.start();
	}

	public void run() {
		target.call(msg);
	}
}

class Synch {
	public static void main(String args[]) {
		Callme target = new Callme();
		Caller ob1 = new Caller(target, "Hello");
		Caller ob2 = new Caller(target, "Synchronized");
		Caller ob3 = new Caller(target, "World");

		// wait for threads to end
		try {
			ob1.t.join();
			ob2.t.join();
			ob3.t.join();
		} catch (InterruptedException e) {
			System.out.println("Interrupted");
		}
	}
}

二.同步语句:

/** NOTES:
 * 在类中创建synchronized方法是一种取得同步的简单有效的方法,但它并不是在所有情况下都有效,为什么?
 * 考虑下面的情况:假设你想同步访问没有设计为多线程访问的类的对象,也就是说,类不使用synchronized方法,
 * 或者更进一步,该类不是由你创建,而是由第三方创建的,你并不能访问它的源代码,因此此时不能在类相应的方法中
 * 增加synchronized关键词.那么怎样同步访问该类的对象呢?幸运的是,这个问题的解决方法非常简单:只需要将对
 * 该类方法的访问置于一个同步块中.
 */
//This program uses a synchronized block.

class Callme2 {
	void call(String msg) {
		System.out.print("[" + msg);
		try {
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			System.out.println("Interrupted");
		}
		System.out.println("]");
	}
}




class Caller2 implements Runnable {
	String msg;

	Callme2 target;

	Thread t;

	public Caller2(Callme2 targ, String s) {
		target = targ;
		msg = s;
		t = new Thread(this);
		t.start();
	}

	// synchronize calls to call()
	public void run() {
		synchronized (target) { // 同步块 synchronized block
			target.call(msg);
		}
	}
}




public class Synch2 {
	public static void main(String args[]) {
		Callme2 target = new Callme2();
		Caller2 ob1 = new Caller2(target, "Hello");
		Caller2 ob2 = new Caller2(target, "Synchronized");
		Caller2 ob3 = new Caller2(target, "World");

		// wait for threads to end
		try {
			ob1.t.join();
			ob2.t.join();
			ob3.t.join();
		} catch (InterruptedException e) {
			System.out.println("Interrupted");
		}
	}
}