对synchronized的一点认识

今天看了一下多线程。
对synchronized这个东东是研究了一上午啊！
首先做一下总结：
对synchronized的使用主要在两方面　
1,synchronized　method()。线程安全，
2,synchronized（object ）。“同步控制块”，不过我更喜欢“临界区”这个叫法。（操作系统里是这样写的，呵呵）！

synchronized　method()。这个嘛，不用说是对类中的方法的锁定，在方法未执行完，但执有该方法线程时间片已到时，锁定该方法，其它线程将不能访问该方法，直到执有此方法的线程释放该方法，其它线程才能访问此方法。

synchronized（object ）则从名字上可看出来是对对象的锁定。是对一个资源的锁定。也即：当一个线程在使用一个对象资源时，其它线程等待，或者说处于阻塞状态，只有该线程释放对（object）的锁定时，才再一次激活。
下面是我自己的测试例子：

java 代码

package synchronizedTest;

public class Student {

    private int age;

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public  void test(int age){
        try {
            this.setAge(age);
            System.out.println(Thread.currentThread().getName()+" Setage:"+age);
            Thread.sleep(500);
            System.out.println(Thread.currentThread().getName()+" Getage:"+this.getAge());
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

java 代码

package synchronizedTest;

import java.util.Random;

public class Test extends Thread {

    private Student student;
    public Test(){
        this.student=new Student();
    }

    public Test(Student student){
        this.student=student;
    }

    public void run(){
        System.out.println(this.getName()+" start...");
        for(int i=0;i<5;i++){
            Random ran=new Random();
            int age=ran.nextInt(60);
            student.test(age);
        }
        System.out.println(this.getName()+" end...");
    }
}

java 代码

package synchronizedTest;

public class Demo {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Student student=new Student();
        Test t1=new Test(student);
        Test t2=new Test(student);
        t1.setName("A");
        t2.setName("B");
        t1.start();
        t2.start();
    }

}

结果如下：
A start...
A Setage:23
B start...
B Setage:53
A Getage:53
A Setage:56
B Getage:56
B Setage:52
A Getage:52
A Setage:2
B Getage:2
B Setage:19
A Getage:19
A Setage:47
B Getage:47
B Setage:2
A Getage:2
A Setage:19
B Getage:19
B Setage:3
B Getage:3
B end...
A Getage:3
A end...
明显上面的例子中发了资源冲突，下面我们对Student类的Test方法加一个线程同步（synchronized）：

java 代码

public synchronized void test(int age){
        try {
            this.setAge(age);
            System.out.println(Thread.currentThread().getName()+" Setage:"+age);
            Thread.sleep(500);
            System.out.println(Thread.currentThread().getName()+" Getage:"+this.getAge());
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

结果如下：
A start...
A Setage:46
B start...
A Getage:46
B Setage:48
B Getage:48
A Setage:56
A Getage:56
B Setage:47
B Getage:47
A Setage:5
A Getage:5
B Setage:2
B Getage:2
A Setage:58
A Getage:58
B Setage:59
B Getage:59
A Setage:0
A Getage:0
A end...
B Setage:17
B Getage:17
B end...
看到我们想要结果了！

我们再继续讨论    synchronized(object)依然用上面的Student类;

java 代码

import java.util.Random;

public class StudentThread extends Thread {

    private Student student;

    public StudentThread(Student student){
        this.student=student;
    }
    public  StudentThread(){
        this.student=new Student();
    }

    public void setStudentAge(int age){
        this.student.setAge(age);
        System.out.println(this.getName()+" set Studnet age:"+this.student.getAge());
    }

    public void printAge(){
        System.out.println(this.getName()+" get Student age:"+this.student.getAge());
    }


     public void  run(){
        System.out.println(this.getName()+" start...");
         test();
    }

     public synchronized void test(){
         try{
                for(int i=0;i<5;i++){

                    Random ran=new Random();
                    int age=ran.nextInt(50);
                    synchronized(student){
                        this.setStudentAge(age);
                        this.sleep(500);
                        this.printAge();
                    }
                }
            }
            catch(Exception e){
                e.printStackTrace();
            }
     }
}

java 代码

public class ThreadDemo {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Student student=new Student();

        StudentThread thread1=new StudentThread(student);
        StudentThread thread2=new StudentThread(student);


        thread1.setName("a");
        thread2.setName("b");

        thread1.start();
        thread2.start();
    }
}

我们再看看结果：
a start...
a set Studnet age:13
b start...
a get Student age:13
b set Studnet age:7
b get Student age:7
a set Studnet age:11
a get Student age:11
b set Studnet age:22
b get Student age:22
a set Studnet age:43
a get Student age:43
b set Studnet age:49
b get Student age:49
a set Studnet age:49
a get Student age:49
b set Studnet age:46
b get Student age:46
a set Studnet age:36
a get Student age:36
b set Studnet age:1
b get Student age:1

评论

2 楼 dennis_zane 2007-11-30  

怎么回复了，还删不了？路过，路过，不好意思

1 楼 heshencao 2007-11-30  

呵呵，JAVA新人。初次发贴！