《Java多线程编程核心技术》读后感（四）
2024-09-04 14:00:21
将任意对象作为对象监视器
synchronized同步代码块还支持任意对象，使用格式为synchronized（非this对象）
package Second;
public class Service {
private String usernameParam;
private String passwordParam;
private String anyString = new String();
public void setUsernamePassword(String username, String password) {
try {
synchronized (anyString) {
System.out.println("线程名称为：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "进入同步块");
usernameParam = username;
Thread.sleep(3000);
passwordParam = password;
System.out.println("线程名称为：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "离开同步块");
}
} catch (InterruptedException e) {
// TODO Auto-generated catch block
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.setUsernamePassword("a", "aa");
}
}
package Second;
public class ThreadB extends Thread {
private Service service;
public ThreadB(Service service) {
super();
this.service = service;
}
@Override
public void run() {
service.setUsernamePassword("b", "bb");
}
}
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();
}
}
package Second;
public class Service {
private String usernameParam;
private String passwordParam;
public void setUsernamePassword(String username, String password) {
try {
String anyString = new String();
synchronized (anyString) {
System.out.println("线程名称为：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "进入同步块");
usernameParam = username;
Thread.sleep(3000);
passwordParam = password;
System.out.println("线程名称为：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "离开同步块");
}
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
package Second;
public class Service {
private String anyString = new String();
public void a() {
try {
synchronized (anyString) {
System.out.println("a begin");
Thread.sleep(3000);
System.out.println("a end");
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
synchronized public void b() {
System.out.println("b begin");
System.out.println("b end");
}
}
package Second;
public class ThreadA extends Thread {
private Service service;
public ThreadA(Service service) {
super();
this.service = service;
}
@Override
public void run() {
service.a();
}
}
package Second;
public class ThreadB extends Thread {
private Service service;
public ThreadB(Service service) {
super();
this.service = service;
}
@Override
public void run() {
service.b();
}
}
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();
}
}
由于对象监视器不同，所以运行结果就是异步的
下面验证多个线程调用同一个方法是随机的
package Second; import java.util.ArrayList;
import java.util.List; public class MyList { private List list = new ArrayList(); synchronized public void add(String username) {
System.out.println("ThreadName=" + Thread.currentThread().getName()
+ "执行了add方法！");
list.add(username);
System.out.println("ThreadName=" + Thread.currentThread().getName()
+ "退出了add方法！");
} synchronized public int getSize() {
System.out.println("ThreadName=" + Thread.currentThread().getName()
+ "执行了getSize方法！");
int sizeValue = list.size();
System.out.println("ThreadName=" + Thread.currentThread().getName()
+ "退出了getSize方法！");
return sizeValue;
} }
package Second;
public class MyThreadA extends Thread {
private MyList list;
public MyThreadA(MyList list) {
super();
this.list = list;
}
@Override
public void run() {
for (int i = 0; i < 100000; i++) {
list.add("threadA" + (i + 1));
}
}
}
package Second;
public class MyThreadB extends Thread {
private MyList list;
public MyThreadB(MyList list) {
super();
this.list = list;
}
@Override
public void run() {
for (int i = 0; i < 100000; i++) {
list.add("threadB" + (i + 1));
}
}
}
package Second;
public class Test {
public static void main(String[] args) {
MyList mylist = new MyList();
MyThreadA a = new MyThreadA(mylist);
a.setName("A");
a.start();
MyThreadB b = new MyThreadB(mylist);
b.setName("B");
b.start();
}
}
下面展示多线程出现脏读的情况
package Second; import java.util.ArrayList;
import java.util.List; public class MyOneList { private List list = new ArrayList(); synchronized public void add(String data) {
list.add(data);
}; synchronized public int getSize() {
return list.size();
}; }
package Second;
public class MyService {
public MyOneList addServiceMethod(MyOneList list, String data) {
try {
synchronized (list) {
if (list.getSize() < 1) {
Thread.sleep(2000);
list.add(data);
}
}
} catch (InterruptedException e) {
e.printStackTrace();
}
return list;
}
}
package Second;
public class MyThread1 extends Thread {
private MyOneList list;
public MyThread1(MyOneList list) {
super();
this.list = list;
}
@Override
public void run() {
MyService msRef = new MyService();
msRef.addServiceMethod(list, "A");
}
}
package Second;
public class MyThread2 extends Thread {
private MyOneList list;
public MyThread2(MyOneList list) {
super();
this.list = list;
}
@Override
public void run() {
MyService msRef = new MyService();
msRef.addServiceMethod(list, "B");
}
}
package Second;
public class Run {
public static void main(String[] args) throws InterruptedException {
MyOneList list = new MyOneList();
MyThread1 thread1 = new MyThread1(list);
thread1.setName("A");
thread1.start();
MyThread2 thread2 = new MyThread2(list);
thread2.setName("B");
thread2.start();
Thread.sleep(6000);
System.out.println("listSize=" + list.getSize());
}
}
"脏读"出现了。出现的原因是两个线程以异步的方式返回list参数的size()大小。解决办法是“”同步化
package Second;
public class MyService {
public MyOneList addServiceMethod(MyOneList list, String data) {
try {
synchronized (list) {
if (list.getSize() < 1) {
Thread.sleep(2000);
list.add(data);
}
}
} catch (InterruptedException e) {
e.printStackTrace();
}
return list;
}
}
由于list参数对象在项目中是一份实例，是单例的，而且也正需要对list参数的getsize()方法做同步的调用，所以就对list参数进行同步处理
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