《Java多线程编程核心技术》读后感（二）

方法内的变量为线程安全

package Second;

public class HasSelfPrivateNum {
    public void addI(String username) {
        try {
            int num = 0;
            if (username.equals("a")) {
                num = 100;
                System.out.println("a set over!");
                Thread.sleep(2000);
            } else {
                num = 200;
                System.out.println("b set over!");
            }
            System.out.println(username + " num=" + num);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

package Second;

public class ThreadA extends Thread {

    private HasSelfPrivateNum numRef;

    public ThreadA(HasSelfPrivateNum numRef) {
        super();
        this.numRef = numRef;
    }

    @Override
    public void run() {
        super.run();
        numRef.addI("a");
    }

}

package Second;

public class ThreadB extends Thread {

    private HasSelfPrivateNum numRef;

    public ThreadB(HasSelfPrivateNum numRef) {
        super();
        this.numRef = numRef;
    }

    @Override
    public void run() {
        super.run();
        numRef.addI("b");
    }

}

package Second;

public class Run {
    public static void main(String[] args) {

        HasSelfPrivateNum numRef = new HasSelfPrivateNum();

        ThreadA athread = new ThreadA(numRef);
        athread.start();

        ThreadB bthread = new ThreadB(numRef);
        bthread.start();

    }
}

实例变量非线程安全

如果对象中有多个实例变量，则运行结果有可能出现交叉的情况

如果对象仅有一个变量，则有可能出现覆盖的情况

package Second;

public class HasSelfPrivateNum {
    private int num = 0;
    public void addI(String username) {
        try {
            if (username.equals("a")) {
                num = 100;
                System.out.println("a set over!");
                Thread.sleep(2000);
            } else {
                num = 200;
                System.out.println("b set over!");
            }
            System.out.println(username + " num=" + num);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

解决方案：

package Second;

public class HasSelfPrivateNum {
    private int num = 0;
    synchronized public void addI(String username) {
        try {
            if (username.equals("a")) {
                num = 100;
                System.out.println("a set over!");
                Thread.sleep(2000);
            } else {
                num = 200;
                System.out.println("b set over!");
            }
            System.out.println(username + " num=" + num);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

实验结论：在两个线程访问同一个对象重点额同步方法时一定是线程安全的

多个对象多个锁

其他代码实现代码如上

package Second;

public class Run {
    public static void main(String[] args) {

        HasSelfPrivateNum numRef1 = new HasSelfPrivateNum();
        HasSelfPrivateNum numRef2 = new HasSelfPrivateNum();

        ThreadA athread = new ThreadA(numRef1);
        athread.start();

        ThreadB bthread = new ThreadB(numRef2);
        bthread.start();

    }
}

上面例子是两个线程分别访问同一个类的两个不同实例的相同名称的同步方法，效果却以异步的方式运行的。本实例由于创建了两个业务对象，在系统中产生出两个锁，所以运行结果是异步的

sychronized方法与锁对象

证明前面讲述的线程锁的是对象

package Second;

public class MyObject {

     public void methodA() {
        try {
            System.out.println("begin methodA threadName="
                    + Thread.currentThread().getName());
            Thread.sleep(500000);
            System.out.println("end");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

package Second;

public class ThreadA extends Thread {

    private MyObject object;

    public ThreadA(MyObject object) {
        super();
        this.object = object;
    }

    @Override
    public void run() {
        super.run();
        object.methodA();
    }

}

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.methodA();
    }

}

package Second;

public class Run {

    public static void main(String[] args) {
        MyObject object = new MyObject();
        ThreadA a = new ThreadA(object);
        a.setName("A");
        ThreadB b = new ThreadB(object);
        b.setName("B");

        a.start();
        b.start();
    }
}

package Second;

public class MyObject {

    synchronized public void methodA() {
        try {
            System.out.println("begin methodA threadName="
                    + Thread.currentThread().getName());
            Thread.sleep(5000);
            System.out.println("end");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

调用关键字synchronized声明的方法一定是在排队运行中，另外需要牢牢记住“共享”两个字，只有共享资源的读写访问才需要同步化，如果不是共享资源，就没有同步的必要

package chapter02.section01.thread_2_1_4.project_2_synchronizedMethodLockObject2;

public class MyObject {

    synchronized public void methodA() {
        try {
            System.out.println("begin methodA threadName="
                    + Thread.currentThread().getName());
            Thread.sleep(5000);
            System.out.println("end endTime=" + System.currentTimeMillis());
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

  public void methodB() {
        try {
            System.out.println("begin methodB threadName="
                    + Thread.currentThread().getName() + " begin time="
                    + System.currentTimeMillis());
            Thread.sleep(5000);
            System.out.println("end");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

package chapter02.section01.thread_2_1_4.project_2_synchronizedMethodLockObject2;

public class ThreadA extends Thread {

    private MyObject object;

    public ThreadA(MyObject object) {
        super();
        this.object = object;
    }

    @Override
    public void run() {
        super.run();
        object.methodA();
    }

}

package chapter02.section01.thread_2_1_4.project_2_synchronizedMethodLockObject2;

public class ThreadB extends Thread {

    private MyObject object;

    public ThreadB(MyObject object) {
        super();
        this.object = object;
    }

    @Override
    public void run() {
        super.run();
        object.methodB();
    }
}

package chapter02.section01.thread_2_1_4.project_2_synchronizedMethodLockObject2;

public class Run {

    public static void main(String[] args) {
        MyObject object = new MyObject();
        ThreadA a = new ThreadA(object);
        a.setName("A");
        ThreadB b = new ThreadB(object);
        b.setName("B");

        a.start();
        b.start();
    }

}

虽然线程A先持有了object对象的锁，但线程B完全可以异步调用非synchronized类型的方法

package chapter02.section01.thread_2_1_4.project_2_synchronizedMethodLockObject2;

public class MyObject {

    synchronized public void methodA() {
        try {
            System.out.println("begin methodA threadName="
                    + Thread.currentThread().getName());
            Thread.sleep(5000);
            System.out.println("end endTime=" + System.currentTimeMillis());
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    synchronized public void methodB() {
        try {
            System.out.println("begin methodB threadName="
                    + Thread.currentThread().getName() + " begin time="
                    + System.currentTimeMillis());
            Thread.sleep(5000);
            System.out.println("end");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

脏读

发生脏读的情况是在读取实例变量时，此值已经被其他线程更改过了

package Second;

public class PublicVar {

    public String username = "A";
    public String password = "AA";

    synchronized public void setValue(String username, String password) {
        try {
            this.username = username;
            Thread.sleep(5000);
            this.password = password;

            System.out.println("setValue method thread name=" + Thread.currentThread().getName() + " username="
                    + username + " password=" + password);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public void getValue() {
        System.out.println("getValue method thread name=" + Thread.currentThread().getName() + " username=" + username
                + " password=" + password);
    }
}

package Second;

public class ThreadA extends Thread {

    private PublicVar publicVar;

    public ThreadA(PublicVar publicVar) {
        super();
        this.publicVar = publicVar;
    }

    @Override
    public void run() {
        super.run();
        publicVar.setValue("B", "BB");
    }

}

package Second;

public class Test {

    public static void main(String[] args) {
        try {
            PublicVar publicVarRef = new PublicVar();
            ThreadA thread = new ThreadA(publicVarRef);
            thread.start();

            Thread.sleep(200);

            publicVarRef.getValue();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }
}

出现脏读是因为getValue()并不是同步的，所以可以在任意时候进行调用

package Second;

public class PublicVar {

    public String username = "A";
    public String password = "AA";

    synchronized public void setValue(String username, String password) {
        try {
            this.username = username;
            Thread.sleep(5000);
            this.password = password;

            System.out.println("setValue method thread name=" + Thread.currentThread().getName() + " username="
                    + username + " password=" + password);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    synchronized public void getValue() {
        System.out.println("getValue method thread name=" + Thread.currentThread().getName() + " username=" + username
                + " password=" + password);
    }
}

synchronized锁重入

也就是在使用synchronized时，当一个线程得到一个对象锁后，再次请求此对象锁时是可以再次得到该对象的锁的。这也证明在一个synchronized方法/块内的内部调用本类的其他synchronized方法/块时，是用户可以得到锁的。

package Second;

public class Service {

    synchronized public void service1() {
        System.out.println("service1");
        service2();
    }

    synchronized public void service2() {
        System.out.println("service2");
        service3();
    }

    synchronized public void service3() {
        System.out.println("service3");
    }

}

package Second;

public class MyThread extends Thread {
    @Override
    public void run() {
        Service service = new Service();
        service.service1();
    }

}

package Second;

public class Run {
    public static void main(String[] args) {
        MyThread t = new MyThread();
        t.start();
    }
}

package Second;

public class Main {

    public int i = 10;

    synchronized public void operateIMainMethod() {
        try {
            i--;
            System.out.println("main print i=" + i);
            Thread.sleep(100);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}

package Second;

public class Sub extends Main {

    synchronized public void operateISubMethod() {
        try {
            while (i > 0) {
                i--;
                System.out.println("sub print i=" + i);
                Thread.sleep(100);
                this.operateIMainMethod();
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

package Second;

public class MyThread extends Thread {
    @Override
    public void run() {
        Sub sub = new Sub();
        sub.operateISubMethod();
    }

}

package Second;

public class Run {
    public static void main(String[] args) {
        MyThread t = new MyThread();
        t.start();
        Main main = new Main();
        System.out.println(main.i);
    }
}

此实验表明，当存在父子类继承关系时，子类是完全可以通过“”可重入锁“”调用父类的同步方法的

出现异常，锁自动释放

当一个线程执行的代码出现异常时，其所持有的锁会自动释放

package Second;

public class Service {
    synchronized public void testMethod() {
        if (Thread.currentThread().getName().equals("a")) {
            System.out.println("ThreadName=" + Thread.currentThread().getName()
                    + " run beginTime=" + System.currentTimeMillis());
            int i = 1;
            while (i == 1) {
                if (("" + Math.random()).substring(0, 8).equals("0.123456")) {
                    System.out.println("ThreadName="
                            + Thread.currentThread().getName()
                            + " run   exceptionTime="
                            + System.currentTimeMillis());
                    Integer.parseInt("a");
                }
            }
        } else {
            System.out.println("Thread B run Time="
                    + System.currentTimeMillis());
        }
    }
}

package Second;

public class ThreadA extends Thread {

    private Service service;

    public ThreadA(Service service) {
        super();
        this.service = service;
    }

    @Override
    public void run() {
        service.testMethod();
    }

}

package Second    ;

public class ThreadB extends Thread {
    private Service service;

    public ThreadB(Service service) {
        super();
        this.service = service;
    }

    @Override
    public void run() {
        service.testMethod();
    }

}

package Second;

public class Test {

    public static void main(String[] args) {
        try {
            Service service = new Service();

            ThreadA a = new ThreadA(service);
            a.setName("a");
            a.start();

            Thread.sleep(500);

            ThreadB b = new ThreadB(service);
            b.setName("b");
            b.start();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

线程a出现异常并释放锁，线程b进入方法正常打印

同步不具有继承性

package Second;

public class Main {

    synchronized public void serviceMethod() {
        try {
            System.out.println("int main 下一步 sleep begin threadName="
                    + Thread.currentThread().getName() + " time="
                    + System.currentTimeMillis());
            Thread.sleep(5000);
            System.out.println("int main 下一步 sleep   end threadName="
                    + Thread.currentThread().getName() + " time="
                    + System.currentTimeMillis());
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

package Second;

public class Sub extends Main {

    @Override
    synchronized public void serviceMethod() {
        try {
            System.out.println("int sub 下一步 sleep begin threadName="
                    + Thread.currentThread().getName() + " time="
                    + System.currentTimeMillis());
            Thread.sleep(5000);
            System.out.println("int sub 下一步 sleep   end threadName="
                    + Thread.currentThread().getName() + " time="
                    + System.currentTimeMillis());
            super.serviceMethod();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}

package Second;

public class MyThreadA extends Thread {

    private Sub sub;

    public MyThreadA(Sub sub) {
        super();
        this.sub = sub;
    }

    @Override
    public void run() {
        sub.serviceMethod();
    }

}

package Second;

public class MyThreadB extends Thread {

    private Sub sub;

    public MyThreadB(Sub sub) {
        super();
        this.sub = sub;
    }

    @Override
    public void run() {
        sub.serviceMethod();
    }
}

所以还要在子类的方法中添加synchronized关键字