《Java多线程编程核心技术》读后感（五）
2024-09-28 21:18:53
下面验证上面三条结论
验证第一条结论：
package Second;
public class MyObject {
}
package Second;
public class Service {
public void testMethod1(MyObject object) {
synchronized (object) {
try {
System.out.println("testMethod1 ____getLock time="
+ System.currentTimeMillis() + " run ThreadName="
+ Thread.currentThread().getName());
Thread.sleep(2000);
System.out.println("testMethod1 releaseLock time="
+ System.currentTimeMillis() + " run ThreadName="
+ Thread.currentThread().getName());
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
package Second;
public class ThreadA extends Thread {
private Service service;
private MyObject object;
public ThreadA(Service service, MyObject object) {
super();
this.service = service;
this.object = object;
}
@Override
public void run() {
super.run();
service.testMethod1(object);
}
}
package Second;
public class ThreadB extends Thread {
private Service service;
private MyObject object;
public ThreadB(Service service, MyObject object) {
super();
this.service = service;
this.object = object;
}
@Override
public void run() {
super.run();
service.testMethod1(object);
}
}
package Second;
public class Run1_1 {
public static void main(String[] args) {
Service service = new Service();
MyObject object = new MyObject();
ThreadA a = new ThreadA(service, object);
a.setName("a");
a.start();
ThreadB b = new ThreadB(service, object);
b.setName("b");
b.start();
}
}
同步的原因是使用了同一个“对象监视器“”。如果使用不同的“”对象监视器“”会出现什么效果呢？见下面
package Second;
public class Run1_2 {
public static void main(String[] args) {
Service service = new Service();
MyObject object1 = new MyObject();
MyObject object2 = new MyObject();
ThreadA a = new ThreadA(service, object1);
a.setName("a");
a.start();
ThreadB b = new ThreadB(service, object2);
b.setName("b");
b.start();
}
}
package Second;
public class Run1_2 {
public static void main(String[] args) {
Service service = new Service();
MyObject object1 = new MyObject();
MyObject object2 = new MyObject();
ThreadA a = new ThreadA(service, object1);
a.setName("a");
a.start();
ThreadB b = new ThreadB(service, object2);
b.setName("b");
b.start();
}
}
验证第2个结论
package Second;
public class MyObject {
synchronized public void speedPrintString() {
System.out.println("speedPrintString ____getLock time="
+ System.currentTimeMillis() + " run ThreadName="
+ Thread.currentThread().getName());
System.out.println("-----------------");
System.out.println("speedPrintString releaseLock time="
+ System.currentTimeMillis() + " run ThreadName="
+ Thread.currentThread().getName());
}
}
package Second;
public class Service {
public void testMethod1(MyObject object) {
synchronized (object) {
try {
System.out.println("testMethod1 ____getLock time="
+ System.currentTimeMillis() + " run ThreadName="
+ Thread.currentThread().getName());
Thread.sleep(5000);
System.out.println("testMethod1 releaseLock time="
+ System.currentTimeMillis() + " run ThreadName="
+ Thread.currentThread().getName());
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
package Second;
public class ThreadA extends Thread {
private Service service;
private MyObject object;
public ThreadA(Service service, MyObject object) {
super();
this.service = service;
this.object = object;
}
@Override
public void run() {
super.run();
service.testMethod1(object);
}
}
package Second;
public class ThreadB extends Thread {
private MyObject object;
public ThreadB(MyObject object) {
super();
this.object = object;
}
@Override
public void run() {
super.run();
object.speedPrintString();
}
}
package Second;
public class Run {
public static void main(String[] args) throws InterruptedException {
Service service = new Service();
MyObject object = new MyObject();
ThreadA a = new ThreadA(service, object);
a.setName("a");
a.start();
Thread.sleep(100);
ThreadB b = new ThreadB(object);
b.setName("b");
b.start();
}
}
验证第3个结论
其他代码与第二个实验相同
package Second;
public class MyObject {
public void speedPrintString() {
synchronized (this) {
System.out.println("speedPrintString ____getLock time="
+ System.currentTimeMillis() + " run ThreadName="
+ Thread.currentThread().getName());
System.out.println("-----------------");
System.out.println("speedPrintString releaseLock time="
+ System.currentTimeMillis() + " run ThreadName="
+ Thread.currentThread().getName());
}
}
}
静态同步synchronized方法与synchronized(class)代码块
是对当前的*.java文件对应的class类进行持锁
package Second;
public class Service {
synchronized public static void printA() {
try {
System.out.println("线程名称为：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "进入printA");
Thread.sleep(3000);
System.out.println("线程名称为：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "离开printA");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
synchronized public static void printB() {
System.out.println("线程名称为：" + Thread.currentThread().getName() + "在"
+ System.currentTimeMillis() + "进入printB");
System.out.println("线程名称为：" + Thread.currentThread().getName() + "在"
+ System.currentTimeMillis() + "离开printB");
}
}
package Second;
public class ThreadA extends Thread {
@Override
public void run() {
Service.printA();
}
}
package Second;
public class ThreadB extends Thread {
@Override
public void run() {
Service.printB();
}
}
package Second;
public class Run {
public static void main(String[] args) {
ThreadA a = new ThreadA();
a.setName("A");
a.start();
ThreadB b = new ThreadB();
b.setName("B");
b.start();
}
}
下面展示synchronized关键字加到非static静态方法上的锁
package Second;
public class Service {
synchronized public static void printA() {
try {
System.out.println("线程名称为：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "进入printA");
Thread.sleep(3000);
System.out.println("线程名称为：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "离开printA");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
synchronized public static void printB() {
System.out.println("线程名称为：" + Thread.currentThread().getName() + "在"
+ System.currentTimeMillis() + "进入printB");
System.out.println("线程名称为：" + Thread.currentThread().getName() + "在"
+ System.currentTimeMillis() + "离开printB");
}
synchronized public void printC() {
System.out.println("线程名称为：" + Thread.currentThread().getName() + "在"
+ System.currentTimeMillis() + "进入printC");
System.out.println("线程名称为：" + Thread.currentThread().getName() + "在"
+ System.currentTimeMillis() + "离开printC");
}
}
package Second;
public class ThreadA extends Thread {
private Service service;
public ThreadA(Service service) {
super();
this.service = service;
}
@Override
public void run() {
service.printA();
}
}
package Second;
public class ThreadB extends Thread {
private Service service;
public ThreadB(Service service) {
super();
this.service = service;
}
@Override
public void run() {
service.printB();
}
}
package Second;
public class ThreadC extends Thread {
private Service service;
public ThreadC(Service service) {
super();
this.service = service;
}
@Override
public void run() {
service.printC();
}
}
异步的原因是持有不同的锁，一个是对象锁，另外一个是class锁，而class锁可以对类的所有对象实例起作用，下面验证
package Second;
public class Service {
synchronized public static void printA() {
try {
System.out.println("线程名称为：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "进入printA");
Thread.sleep(3000);
System.out.println("线程名称为：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "离开printA");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
synchronized public static void printB() {
System.out.println("线程名称为：" + Thread.currentThread().getName() + "在"
+ System.currentTimeMillis() + "进入printB");
System.out.println("线程名称为：" + Thread.currentThread().getName() + "在"
+ System.currentTimeMillis() + "离开printB");
}
}
package Second;
public class ThreadA extends Thread {
private Service service;
public ThreadA(Service service) {
super();
this.service = service;
}
@Override
public void run() {
service.printA();
}
}
package Second;
public class ThreadB extends Thread {
private Service service;
public ThreadB(Service service) {
super();
this.service = service;
}
@Override
public void run() {
service.printB();
}
}
package Second;
public class Run {
public static void main(String[] args) {
Service service1 = new Service();
Service service2 = new Service();
ThreadA a = new ThreadA(service1);
a.setName("A");
a.start();
ThreadB b = new ThreadB(service2);
b.setName("B");
b.start();
}
}
同步synchronized（class）代码块的作用其实和synchronized static方法的作用是一样的。下面测试
package Second;
public class Service {
public static void printA() {
synchronized (Service.class) {
try {
System.out.println("线程名称为：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "进入printA");
Thread.sleep(3000);
System.out.println("线程名称为：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "离开printA");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static void printB() {
synchronized (Service.class) {
System.out.println("线程名称为：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "进入printB");
System.out.println("线程名称为：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "离开printB");
}
}
}
package Second;
public class ThreadA extends Thread {
private Service service;
public ThreadA(Service service) {
super();
this.service = service;
}
@Override
public void run() {
service.printA();
}
}
package Second;
public class ThreadB extends Thread {
private Service service;
public ThreadB(Service service) {
super();
this.service = service;
}
@Override
public void run() {
service.printB();
}
}
package Second;
public class Run {
public static void main(String[] args) {
Service service1 = new Service();
Service service2 = new Service();
ThreadA a = new ThreadA(service1);
a.setName("A");
a.start();
ThreadB b = new ThreadB(service2);
b.setName("B");
b.start();
}
}
数据类型String的常量池特性
package Second;
public class Service {
public static void print(String stringParam) {
try {
synchronized (stringParam) {
while (true) {
System.out.println(Thread.currentThread().getName());
Thread.sleep(1000);
}
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
package Second;
public class ThreadA extends Thread {
private Service service;
public ThreadA(Service service) {
super();
this.service = service;
}
@Override
public void run() {
service.print("AA");
}
}
package Second;
public class ThreadB extends Thread {
private Service service;
public ThreadB(Service service) {
super();
this.service = service;
}
@Override
public void run() {
service.print("AA");
}
}
package Second;
public class Run {
public static void main(String[] args) {
Service service = new Service();
ThreadA a = new ThreadA(service);
a.setName("A");
a.start();
ThreadB b = new ThreadB(service);
b.setName("B");
b.start();
}
}
出现这样的情况就是因为String的两个值都是AA,两个线程持有相同的锁，所以造成线程B不能执行。这就是String常量池所带来的问题。
因此在大多数情况下，同步synchronized代码块都不使用String作为锁对象，而改用其他的，比如new object（）实例化一个object对象，但它并不放入缓存中。
package Second;
public class Service {
public static void print(Object object) {
try {
synchronized (object) {
while (true) {
System.out.println(Thread.currentThread().getName());
Thread.sleep(1000);
}
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
package Second;
public class ThreadA extends Thread {
private Service service;
public ThreadA(Service service) {
super();
this.service = service;
}
@Override
public void run() {
service.print(new Object());
}
}
package Second;
public class ThreadB extends Thread {
private Service service;
public ThreadB(Service service) {
super();
this.service = service;
}
@Override
public void run() {
service.print(new Object());
}
}
package Second;
public class Run {
public static void main(String[] args) {
Service service = new Service();
ThreadA a = new ThreadA(service);
a.setName("A");
a.start();
ThreadB b = new ThreadB(service);
b.setName("B");
b.start();
}
}
交替打印是因为持有的锁不是同一个
同步synchronized方法无限等待与解决
同步方法容易造成死循环
package Second;
public class Service {
synchronized public void methodA() {
Object object1 = new Object();
System.out.println("methodA begin");
boolean isContinueRun = true;
while (isContinueRun) {
}
System.out.println("methodA end");
}
synchronized public void methodB() {
Object object2 = new Object();
System.out.println("methodB begin");
System.out.println("methodB end");
}
}
package Second;
public class ThreadA extends Thread {
private Service service;
public ThreadA(Service service) {
super();
this.service = service;
}
@Override
public void run() {
service.methodA();
}
}
package Second;
public class ThreadB extends Thread {
private Service service;
public ThreadB(Service service) {
super();
this.service = service;
}
@Override
public void run() {
service.methodB();
}
}
package Second;
public class Run {
public static void main(String[] args) {
Service service = new Service();
ThreadA athread = new ThreadA(service);
athread.start();
ThreadB bthread = new ThreadB(service);
bthread.start();
}
}
线程B永远得不到运行的机会，锁死了
package Second;
public class Service {
public void methodA() {
Object object1 = new Object();
synchronized (object1) {
System.out.println("methodA begin");
boolean isContinueRun = true;
while (isContinueRun) {
}
System.out.println("methodA end");
}
}
public void methodB() {
Object object2 = new Object();
synchronized (object2) {
System.out.println("methodB begin");
System.out.println("methodB end");
}
}
}
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