《Java多线程编程核心技术》读后感（八）
2024-09-08 05:25:46
不使用等待/通知机制实现线程间通信
使用sleep()结合while(true)死循环来实现多个线程间通信
package Third; import java.util.ArrayList;
import java.util.List; public class MyList { private List list = new ArrayList(); public void add() {
list.add("高洪岩");
} public int size() {
return list.size();
} }
package Third;
public class ThreadA extends Thread {
private MyList list;
public ThreadA(MyList list) {
super();
this.list = list;
}
@Override
public void run() {
try {
for (int i = 0; i < 10; i++) {
list.add();
System.out.println("添加了" + (i + 1) + "个元素");
Thread.sleep(1000);
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
package Third;
public class ThreadB extends Thread {
private MyList list;
public ThreadB(MyList list) {
super();
this.list = list;
}
@Override
public void run() {
try {
while (true) {
if (list.size() == 5) {
System.out.println("==5了，线程b要退出了！");
throw new InterruptedException();
}
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
package Third;
public class Test {
public static void main(String[] args) {
MyList service = new MyList();
ThreadA a = new ThreadA(service);
a.setName("A");
a.start();
ThreadB b = new ThreadB(service);
b.setName("B");
b.start();
}
}
虽然两个线程实现了通信，但有一个弊端是，线程ThreadB.java不停地通过while语轮询机制来检测某一个条件，这样会浪费CPU资源。
如果轮询的时间间隔小，更浪费cpu资源；如果轮询时间间隔大，有可能会娶不到想要的数据。所以就需要有一种机制来减少CPU的资源浪费，而且还可以实现在多个线程间通信，它就是“”wait/notify“”机制。
什么是等待/通信机制
等待/通知机制的实现
package Third;
public class Test1 {
public static void main(String[] args) {
try {
String newString = new String("");
newString.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
出现异常的原因是没有“”对象监视器“”，也就是没有同步加锁
package Third;
public class Test2 {
public static void main(String[] args) {
try {
String lock = new String();
System.out.println("syn上面");
synchronized (lock) {
System.out.println("syn第一行");
lock.wait();
System.out.println("wait下的代码！");
}
System.out.println("syn下面的代码");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
线程不能永远等待下去，那样程序就停止不前，不继续向下运行了。如何使呈现wait状态的线程继续运行？答案是notify()
package Third;
public class MyThread1 extends Thread {
private Object lock;
public MyThread1(Object lock) {
super();
this.lock = lock;
}
@Override
public void run() {
try {
synchronized (lock) {
System.out.println("开始 wait time=" + System.currentTimeMillis());
lock.wait();
System.out.println("结束 wait time=" + System.currentTimeMillis());
}
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
package Third;
public class MyThread2 extends Thread {
private Object lock;
public MyThread2(Object lock) {
super();
this.lock = lock;
}
@Override
public void run() {
synchronized (lock) {
System.out.println("开始notify time=" + System.currentTimeMillis());
lock.notify();
System.out.println("结束notify time=" + System.currentTimeMillis());
}
}
}
package Third;
public class MyThread2 extends Thread {
private Object lock;
public MyThread2(Object lock) {
super();
this.lock = lock;
}
@Override
public void run() {
synchronized (lock) {
System.out.println("开始notify time=" + System.currentTimeMillis());
lock.notify();
System.out.println("结束notify time=" + System.currentTimeMillis());
}
}
}
下面实现前面的size()=5的实验
package Third; import java.util.ArrayList;
import java.util.List; public class MyList { private static List list = new ArrayList(); public static void add() {
list.add("anyString");
} public static int size() {
return list.size();
} }
package Third;
public class ThreadA extends Thread {
private Object lock;
public ThreadA(Object lock) {
super();
this.lock = lock;
}
@Override
public void run() {
try {
synchronized (lock) {
if (MyList.size() != 5) {
System.out.println("wait begin "
+ System.currentTimeMillis());
lock.wait();
System.out.println("wait end "
+ System.currentTimeMillis());
}
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
package Third;
public class ThreadB extends Thread {
private Object lock;
public ThreadB(Object lock) {
super();
this.lock = lock;
}
@Override
public void run() {
try {
synchronized (lock) {
for (int i = 0; i < 10; i++) {
MyList.add();
if (MyList.size() == 5) {
lock.notify();
System.out.println("已发出通知！");
}
System.out.println("添加了" + (i + 1) + "个元素!");
Thread.sleep(1000);
}
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
package Third;
public class Run {
public static void main(String[] args) {
try {
Object lock = new Object();
ThreadA a = new ThreadA(lock);
a.start();
Thread.sleep(50);
ThreadB b = new ThreadB(lock);
b.start();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
日志信息wait end在最后输出，这也说明notify（）执行后并不立即释放锁
方法wait（）锁释放与notify（）锁不释放
当方法wait（）被执行后，锁被自动释放，但执行完notify（）方法，锁却不自动释放
package Third;
public class Service {
public void testMethod(Object lock) {
try {
synchronized (lock) {
System.out.println("begin wait()");
lock.wait();
System.out.println(" end wait()");
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
package Third;
public class ThreadA extends Thread {
private Object lock;
public ThreadA(Object lock) {
super();
this.lock = lock;
}
@Override
public void run() {
Service service = new Service();
service.testMethod(lock);
}
}
package Third;
public class ThreadB extends Thread {
private Object lock;
public ThreadB(Object lock) {
super();
this.lock = lock;
}
@Override
public void run() {
Service service = new Service();
service.testMethod(lock);
}
}
package Third;
public class Test {
public static void main(String[] args) {
Object lock = new Object();
ThreadA a = new ThreadA(lock);
a.start();
ThreadB b = new ThreadB(lock);
b.start();
}
}
下面验证方法notify()被执行后，不释放锁
package Third;
public 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();
}
}
}
package Third;
public class ThreadA extends Thread {
private Object lock;
public ThreadA(Object lock) {
super();
this.lock = lock;
}
@Override
public void run() {
Service service = new Service();
service.testMethod(lock);
}
}
package Third;
public class NotifyThread extends Thread {
private Object lock;
public NotifyThread(Object lock) {
super();
this.lock = lock;
}
@Override
public void run() {
Service service = new Service();
service.synNotifyMethod(lock);
}
}
package Third;
public class synNotifyMethodThread extends Thread {
private Object lock;
public synNotifyMethodThread(Object lock) {
super();
this.lock = lock;
}
@Override
public void run() {
Service service = new Service();
service.synNotifyMethod(lock);
}
}
package Third;
public class Test {
public static void main(String[] args) throws InterruptedException {
Object lock = new Object();
ThreadA a = new ThreadA(lock);
a.start();
NotifyThread notifyThread = new NotifyThread(lock);
notifyThread.start();
synNotifyMethodThread c = new synNotifyMethodThread(lock);
c.start();
}
}
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