介绍一下线程中基本的方法使用

线程睡眠sleep()

Thread.sleep(毫秒);我们可以通过sleep方法设置让线程睡眠。可以看到sleep是个静态方法

public static native void sleep(long var0) throws InterruptedException;

    try {System.out.println(new Date().getSeconds());Thread.sleep(5000);System.out.println(new Date().getSeconds());} catch (InterruptedException e) {e.printStackTrace();}

setDaemon守护线程

非守护线程停止，那么守护线程自动退出

    public static void main(String[] args) {Thread thread1 = new Thread() {@Overridepublic void run() {super.run();for(int i = 0; i < 5; i ++) {System.out.println("非守护线程");}}};Thread thread2 = new Thread() {@Overridepublic void run() {for(int i = 0; i < 200; i ++) {System.out.println("守护线程");}}};thread2.setDaemon(true);thread1.start();thread2.start();}

可以很明显的看到thread2本应该执行200次输出，但是这里只输出了几行。因为当thread1执行完毕后，thread2作为守护线程就自动停止了。

多线程join

如果执行了join方法，那么停止当前线程，先跑执行了join()的线程。相当于插队执行。如下，在执行thread2线程的时候，如果i==20的时候，则让thread1插队先执行

    public static void main(String[] args) {final Thread thread1 = new Thread() {@Overridepublic void run() {super.run();for(int i = 0; i < 500; i ++) {System.out.println("thread1---" + i);}}};Thread thread2 = new Thread() {@Overridepublic void run() {for(int i = 0; i < 200; i ++) {if (i == 20) {try {//插队执行thread1.join();} catch (InterruptedException e) {e.printStackTrace();}}System.out.println(i);}}};thread1.start();thread2.start();}

join()方法也可以传参数long 毫秒 join(毫秒)
表示让执行join的线程，插队执行XXX毫秒，过了时间后，两个线程交替执行

   public static void main(String[] args) {final Thread thread1 = new Thread() {@Overridepublic void run() {super.run();for(int i = 0; i < 500; i ++) {System.out.println("thread1---" + i);}}};Thread thread2 = new Thread() {@Overridepublic void run() {for(int i = 0; i < 200; i ++) {if (i == 20) {try {//插队执行１毫秒thread1.join(1);} catch (InterruptedException e) {e.printStackTrace();}}System.out.println(i);}}};thread1.start();thread2.start();}

yeild 礼让线程

yeild会让出cpu,让其他线程执行

    public static void main(String[] args) {final Thread thread1 = new Thread() {@Overridepublic void run() {super.run();for(int i = 0; i < 500; i ++) {System.out.println( getName() + "---" + i);}}};Thread thread2 = new Thread() {@Overridepublic void run() {for(int i = 0; i < 200; i ++) {if (i % 5 == 0) {Thread.yield();}System.out.println(getName() + "---" + i);}}};thread1.start();thread2.start();}

setPriority给线程设置优先级

默认优先级是5 最小１，最大10
越大优先级越高

public static void main(String[] args) {final Thread thread1 = new Thread() {@Overridepublic void run() {super.run();for(int i = 0; i < 500; i ++) {System.out.println( getName() + "---" + i);}}};Thread thread2 = new Thread() {@Overridepublic void run() {for(int i = 0; i < 500; i ++) {System.out.println(getName() + "---" + i);}}};//设置最大的线程优先级最大为10thread1.setPriority(Thread.MIN_PRIORITY);//设置最小的线程优先级，最小为１thread2.setPriority(Thread.MAX_PRIORITY);thread1.start();thread2.start();}

synchronized

同步代码块

当多线程并发，多段代码同时执行的时候。希望在执行其中代码的时候，cpu不切换线程

不用synchronized的情况

我们来看一下不用synchronized的情况会发生什么

public class ThreadSynchronied {public static void main(String[] args) {final Say say = new Say();Thread thread1 = new Thread() {@Overridepublic void run() {for (int i = 0 ; i < 10000 ; i ++) {say.say();}}};Thread thread2 = new Thread() {@Overridepublic void run() {for (int i = 0 ; i < 10000 ; i ++) {say.say1();}}};//设置最大的线程优先级最大为10thread1.setPriority(Thread.MIN_PRIORITY);//设置最小的线程优先级，最小为１thread2.setPriority(Thread.MAX_PRIORITY);thread1.start();thread2.start();}
}class Say {void say() {System.out.print("s ");System.out.print("a ");System.out.print("y ");System.out.print("h ");System.out.print("e ");System.out.print("l ");System.out.print("l ");System.out.println("o");}void say1() {System.out.print("1 ");System.out.print("2 ");System.out.print("3 ");System.out.print("4 ");System.out.print("5 ");System.out.print("6 ");System.out.print("7 ");System.out.println("8");}
}

我们发现有些输出并没有打印全，在执行线程thread1的过程中，cpu被thread2抢占。这种情况下，肯定是不符合我们的业务逻辑的。所以我们要保证线程执行了一个完整的方法后，cpu才会被其他线程抢占

使用synchronized

public class ThreadSynchronied {public static void main(String[] args) {final Say say = new Say();Thread thread1 = new Thread() {@Overridepublic void run() {for (int i = 0 ; i < 10000 ; i ++) {say.say();}}};Thread thread2 = new Thread() {@Overridepublic void run() {for (int i = 0 ; i < 10000 ; i ++) {say.say1();}}};//设置最大的线程优先级最大为10thread1.setPriority(Thread.MIN_PRIORITY);//设置最小的线程优先级，最小为１thread2.setPriority(Thread.MAX_PRIORITY);thread1.start();thread2.start();}
}class Say {String s = "hahaah";void say() {synchronized (s) {System.out.print("s ");System.out.print("a ");System.out.print("y ");System.out.print("h ");System.out.print("e ");System.out.print("l ");System.out.print("l ");System.out.println("o");}}void say1() {synchronized (s) {System.out.print("1 ");System.out.print("2 ");System.out.print("3 ");System.out.print("4 ");System.out.print("5 ");System.out.print("6 ");System.out.print("7 ");System.out.println("8");}}
}

使用synchronized同步代码块后，就发现不会出现上述情况了

同步方法

public class ThreadSynchroniedMethod {public static void main(String[] args) {final Say say = new Say();Thread thread1 = new Thread() {@Overridepublic void run() {for (int i = 0 ; i < 10000 ; i ++) {say.say();}}};Thread thread2 = new Thread() {@Overridepublic void run() {for (int i = 0 ; i < 10000 ; i ++) {say.say1();}}};//设置最大的线程优先级最大为10thread1.setPriority(Thread.MIN_PRIORITY);//设置最小的线程优先级，最小为１thread2.setPriority(Thread.MAX_PRIORITY);thread1.start();thread2.start();}
}class Say {//在方法上加锁static synchronized void say() {System.out.print("s ");System.out.print("a ");System.out.print("y ");System.out.print("h ");System.out.print("e ");System.out.print("l ");System.out.print("l ");System.out.println("o");}static void say1() {synchronized (Say.class) {System.out.print("1 ");System.out.print("2 ");System.out.print("3 ");System.out.print("4 ");System.out.print("5 ");System.out.print("6 ");System.out.print("7 ");System.out.println("8");}}
}

同步方法指的就是在方法上加锁

静态同步方法的所对象是该类的字节码对象
非静态的同步方法锁对象是this

多个线程使用同一资源锁，容易造成死锁

什么是死锁?

死锁是指两个或两个以上的进程在执行过程中，由于竞争资源或者由于彼此通信而造成的一种阻塞的现象，若无外力作用，它们都将无法推进下去。此时称系统处于死锁状态或系统产生了死锁，这些永远在互相等待的进程称为死锁进程。

线程安全类

Vector
StringBuffer
HashTable

线程不安全

ArrayList
StringBuilder
HashSet

java.util.Collections中有synchronizedList等方法，支持我们把线程不安全的集合转成线程安全的

学习笔记

多次启动一个线程是非法的