《Java多线程编程核心技术》读后感（十二）

类ThreadLocal的使用

主要解决的是每个线程绑定自己的值，可以将ThreadLocal类比喻成全局存放数据的盒子，盒子中可以存储每个线程私有数据。

类ThreadLocal解决的是变量在不同线程间的隔离线，也就是不同线程拥有自己的值，不同线程中的值是可以放入ThreadLocal类中进行保存的

方法get()与null

package Third;

public class Run {
    public static ThreadLocal tl = new ThreadLocal();

    public static void main(String[] args) {
        if (tl.get() == null) {
            System.out.println("从未放过值");
            tl.set("我的值");
        }
        System.out.println(tl.get());
        System.out.println(tl.get());
    }

}

验证线程变量的隔离性

package Third;

public class ThreadA extends Thread {

    @Override
    public void run() {
        try {
            for (int i = 0; i < 100; i++) {
                if (Tools.tl.get() == null) {
                    Tools.tl.set("ThreadA" + (i + 1));
                } else {
                    System.out.println("ThreadA get Value=" + Tools.tl.get());
                }
                Thread.sleep(200);
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

package Third;

public class ThreadB extends Thread {

    @Override
    public void run() {
        try {
            for (int i = 0; i < 100; i++) {
                if (Tools.tl.get() == null) {
                    Tools.tl.set("ThreadB" + (i + 1));
                } else {
                    System.out.println("ThreadB get Value=" + Tools.tl.get());
                }
                Thread.sleep(200);
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

package Third;

public class Tools {

    public static ThreadLocal tl = new ThreadLocal();

}

package Third;

public class Run {

    public static void main(String[] args) {

        try {
            ThreadA a = new ThreadA();
            ThreadB b = new ThreadB();
            a.start();
            b.start();

            for (int i = 0; i < 100; i++) {
                if (Tools.tl.get() == null) {
                    Tools.tl.set("Main" + (i + 1));
                } else {
                    System.out.println("Main get Value=" + Tools.tl.get());
                }
                Thread.sleep(200);
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

    }

}

每个线程取出自己的数据

package Third;

import java.util.Date;

public class ThreadA extends Thread {

    @Override
    public void run() {
        try {
            for (int i = 0; i < 20; i++) {
                if (Tools.tl.get() == null) {
                    Tools.tl.set(new Date());
                }
                System.out.println("A " + Tools.tl.get().getTime());
                Thread.sleep(100);
            }
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}

package Third;

import java.util.Date;

public class ThreadB extends Thread {

    @Override
    public void run() {
        try {
            for (int i = 0; i < 20; i++) {
                if (Tools.tl.get() == null) {
                    Tools.tl.set(new Date());
                }
                System.out.println("B " + Tools.tl.get().getTime());
                Thread.sleep(100);
            }
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}

package Third;

import java.util.Date;

public class Tools {

    public static ThreadLocal<Date> tl = new ThreadLocal<Date>();

}

package Third;

public class Run {

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

            Thread.sleep(1000);

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

    }

}

怎么实现第一次调用get()不返回null呢？也就是具有默认值的效果

解决get()返回null问题

package Third;

public class ThreadLocalExt extends ThreadLocal {
    @Override
    protected Object initialValue() {
        return "我是默认值 第一次get不再为null";
    }
}

package Third;

public class Run {
    public static ThreadLocalExt tl = new ThreadLocalExt();

    public static void main(String[] args) {
        if (tl.get() == null) {
            System.out.println("从未放过值");
            tl.set("我的值");
        }
        System.out.println(tl.get());
        System.out.println(tl.get());
    }

}

此案例仅仅证明main线程有自己的值，那其他线程呢？

再次验证线程变量的隔离性

package Third;

public class Tools {
    public static ThreadLocalExt tl = new ThreadLocalExt();
}

package Third;

import java.util.Date;

public class ThreadLocalExt extends ThreadLocal {
    @Override
    protected Object initialValue() {
        return new Date().getTime();
    }
}

package Third;

public class ThreadA extends Thread {

    @Override
    public void run() {
        try {
            for (int i = 0; i < 10; i++) {
                System.out.println("在ThreadA线程中取值=" + Tools.tl.get());
                Thread.sleep(100);
            }
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}

package Third;

public class Run {

    public static void main(String[] args) {
        try {
            for (int i = 0; i < 10; i++) {
                System.out.println("       在Main线程中取值=" + Tools.tl.get());
                Thread.sleep(100);
            }
            Thread.sleep(5000);
            ThreadA a = new ThreadA();
            a.start();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}