一、等待/通知机制
1、wait()/notify()
方法wait()的作用是使当前执行代码的线程进行等待,wait()方法是Object类的方法,该方法用来将当前线程置入“预执行队列”中,并且在wait()所在的代码行处停止执行,直到接到通知或被中断为止。在调用wait()方法之前,线程必须获得该对象的对象级别锁,即只能在同步方法或者同步块中调用wait()方法。如果调用wait()时没有持有适当的锁,则抛出IllegalMonitorStateException,它是RuntimeException的一个子类,因此不需要try-catch语句进行捕捉异常。
方法notify()的作用是用来通知那些可能等待该对象的对象锁的其他线程,如果有多个线程等待,则由线程规划器随机挑选出其中一个呈wait状态的线程,对其发出通知notify,并 使它等待获取该对象的对象锁。notify()也要在同步方法或者同步块中调用,即在调用前,线程也必须获得该对象的对象级别锁。如果调用notify()时没有持有适当的锁,也会抛出IllegalMonitorStateException 异常。
调用notify()一次只随机通知一个线程进行唤醒。notifyAll()方法可以唤醒全部线程。
注意:
(1)当线程呈wait()状态时,调用线程对象的interrupt()方法会出现InterruptedException异常。
(2)在执行同步块代码的过程中,遇到异常而导致线程终止,锁也会被释放。
(3)在执行wait()方法后,当前线程会立即释放锁,但在执行notify()方法后,当前线程不会马上释放该对象的锁,要等到执行notify()方法的线程将程序执行完,也就是退出synchronized代码块后,当前线程才会释放锁。代码示例如下:
class Service{
public void testMethod(Object lock){
try{
synchronized(lock){
System.out.println("begin wait() ThreadName = " + Thread.currentThread().getName());
lock.wait();
System.out.println(" end wait() ThreadName = " + Thread.currentThread().getName());
}
}catch(InterruptedException e){
e.printStackTrace();
}
}
public void synNotifyMethod(Object lock){
try{
synchronized(lock){
System.out.println("begin notify() ThreadName = " + Thread.currentThread().getName() + " time = "+System.currentTimeMillis());
lock.notify();
Thread.sleep(5000);
System.out.println(" end notify() ThreadName = " + Thread.currentThread().getName() + " time = "+System.currentTimeMillis());
}
}catch(InterruptedException e){
e.printStackTrace();
}
}
}
class Mythread extends Thread{
private Object lock;
public Mythread(Object lock){
this.lock = lock;
}
@Override
public void run(){
Service service = new Service();
service.testMethod(lock);
}
}
class NotifyThread extends Thread{
private Object lock;
public NotifyThread(Object lock){
this.lock = lock;
}
@Override
public void run(){
Service service = new Service();
service.synNotifyMethod(lock);
}
}
public class TestClass{
public static void main(String[] args) throws InterruptedException{
Object lock = new Object();
Mythread t = new Mythread(lock);
t.start();
NotifyThread notifyThread = new NotifyThread(lock);
notifyThread.start();
}
}
程序运行结果如下:
wait(long)方法的功能是等待某一时间内是否有线程对锁进行唤醒,如果超过这个时间则自动唤醒。
2、通过管道进行线程间通信
管道流是一种特殊的流,用于在不同线程间直接传送数据。一个线程发送数据到输出管道,另一个线程从输入管道中读数据。通过使用管道,实现不同线程间的通信,而无须借助于类似临时文件之类的东西。
在Java的JDK中提供了4个类来使线程间可以进行通信:
(1)PipedInputStream和PipedOutputStream
(2)PipedReader和PipedWriter
以下代码是PipedReader和PipedWriter(PipedInputStream和PipedOutputStream的用法是一样的)使用的示例代码:
import java.io.IOException;
import java.io.PipedReader;
import java.io.PipedWriter;
class WriteData{
public void writeMethod(PipedWriter out){
try{
System.out.println("write:");
for(int i = 0 ;i < 10; i++){
String outData = " " + i;
out.write(outData);
System.out.print(outData);
}
System.out.println();
out.close();
}catch(IOException e){
e.printStackTrace();
}
}
}
class ReadData{
public void readMethod(PipedReader input){
try{
System.out.println("read:");
char[] byteArray = new char[20];
int readLength = input.read(byteArray);
while(readLength != -1){
String newData = new String(byteArray,0,readLength);
System.out.print(newData);
readLength = input.read(byteArray);
}
System.out.println();
input.close();
}catch(IOException e){
e.printStackTrace();
}
}
}
class ThreadWrite extends Thread{
private WriteData write;
private PipedWriter out;
public ThreadWrite(WriteData write,PipedWriter out){
this.write = write;
this.out = out;
}
@Override
public void run(){
write.writeMethod(out);
}
}
class ThreadRead extends Thread{
private ReadData read;
private PipedReader input;
public ThreadRead(ReadData read,PipedReader input){
this.read = read;
this.input = input;
}
@Override
public void run(){
read.readMethod(input);
}
}
public class TestClass{
public static void main(String[] args){
try{
WriteData writeData = new WriteData();
ReadData readData = new ReadData();
PipedReader reader = new PipedReader();
PipedWriter writer = new PipedWriter();
writer.connect(reader);
ThreadRead read = new ThreadRead(readData,reader);
read.start();
ThreadWrite write = new ThreadWrite(writeData,writer);
write.start();
}catch(IOException e){
e.printStackTrace();
}
}
}
下图是运行结果:
二、方法join的使用
方法join()的作用是使所属的线程对象正常执行run()方法中的任务,而使当前线程z进行无限期的阻塞,等待线程x销毁后再继续线程z后面的代码。一以下代码可以实现主线程等待子线程执行完毕再打印:
class MyThread extends Thread{
@Override
public void run(){
try {
int secondValue = (int)(Math.random() * 10000);
Thread.sleep(secondValue);
System.out.println(secondValue);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
public class TestClass{
public static void main(String[] args){
try{
MyThread threadTest = new MyThread();
threadTest.start();
threadTest.join();
System.out.println("我想当threadTest对象执行完毕后我再执行,我做到了!");
}catch(InterruptedException e){
e.printStackTrace();
}
}
}
主线程会在子线程打印出随机时间之后在打印:
在join过程中,如果当前线程对象被中断,则当前线程出现异常,其他线程正常执行。
join(long)与sleep(long)的区别:
方法join(long)的功能在内部是使用wait(long)方法来实现的,所以join(long)方法具有释放锁的特点。而Thread.sleep(long)方法却不释放锁。如下代码示例可以展现两者的区别:
class ThreadB extends Thread{
@Override
public void run(){
try{
System.out.println(" b run begin timer = " + System.currentTimeMillis());
Thread.sleep(5000);
System.out.println(" b run end timer = " + System.currentTimeMillis());
}catch(InterruptedException e){
e.printStackTrace();
}
}
synchronized public void bService(){
System.out.println("打印了bService timer = " + System.currentTimeMillis());
}
}
class ThreadA extends Thread{
private ThreadB b;
public ThreadA(ThreadB b){
this.b = b;
}
@Override
public void run(){
try{
synchronized(b){
b.start();
Thread.sleep(6000);
// b.join(6000);
}
}catch(InterruptedException e){
e.printStackTrace();
}
}
}
class ThreadC extends Thread{
private ThreadB b;
public ThreadC(ThreadB b){
this.b = b;
}
@Override
public void run(){
b.bService();
}
}
public class TestClass{
public static void main(String[] args){
ThreadB b = new ThreadB();
ThreadA a = new ThreadA(b);
a.start();
ThreadC c = new ThreadC(b);
c.start();
}
}
由于线程ThreadA使用Thread.sleep(long)方法一直持有ThreadB对象的锁,时间达到6秒,所以线程ThreadC只有在ThreadA时间到达6秒释放ThreadB的锁时,才可以调用ThreadB的同步方法synchronized public void bService()。把上面代码的Thread.sleep(6000);换成b.join(6000),则结果如下所示:
使用join方法后,由于线程ThreadA释放了ThreadB的锁,所以线程ThreadC可以调用ThreadB中的同步方法synchronized public void bService()。
三、类ThreadLocal的使用
类ThreadLocal主要解决的就是每个线程绑定自己的值。ThreadLocal的get()方法放回与本线程相关的值,set()方法将与本线程相关的值绑定。第一次调用ThreadLocal类的get()方法返回值是null。
四、类InheritableThreadLocal的使用
使用InheritableThreadLocal类可以让子线程从父线程中取得值。如下代码所示:
class InheritableThreadLocalExt extends InheritableThreadLocal<Object>{
@Override
protected Object initialValue(){
return "你好!";
}
}
class ThreadA extends Thread{
@Override
public void run(){
System.out.println("ThreadA中的值=" + TestClass.ext.get());
}
}
public class TestClass{
public static InheritableThreadLocalExt ext = new InheritableThreadLocalExt();
public static void main(String[] args){
System.out.println("Main中的值=" + TestClass.ext.get());
ThreadA a = new ThreadA();
a.setName("ThreadA");
a.start();
}
}
运行结果如下:
注意:如果子线程在取值的同时,主线程将InheritableThreadLocal中的值进行更改,那么子线程取到的值还是旧值。