Java线程:深入ThreadLocal
 
ThreadLocal与线程成员变量还有区别,ThreadLocal该类提供了线程局部变量。这个局部变量与一般的成员变量不一样,ThreadLocal的变量在被多个线程使用时候,每个线程只能拿到该变量的一个副本,这是Java API中的描述,通过阅读API源码,发现并非副本,副本什么概念?克隆品? 或者是别的样子,太模糊。
 
准确的说,应该是ThreadLocal类型的变量内部的注册表(Map<Thread,T>)发生了变化,但ThreadLocal类型的变量本身的确是一个,这才是本质!
 
下面就做个例子:
 
一、标准例子
 
定义了MyThreadLocal类,创建它的一个对象tlt,分别给四个线程使用,结果四个线程tlt变量并没有出现共用现象,二是各用各的,这说明,四个线程使用的是tlt的副本(克隆品)。
 
/**
* 使用了ThreadLocal的类
*
* @author leizhimin 2010-1-5 10:35:27
*/

public class MyThreadLocal {
        //定义了一个ThreadLocal变量,用来保存int或Integer数据
        private ThreadLocal<Integer> tl = new ThreadLocal<Integer>() {
                @Override
                protected Integer initialValue() {
                        return 0;
                }
        };

        public Integer getNextNum() {
                //将tl的值获取后加1,并更新设置t1的值
                tl.set(tl.get() + 1);
                return tl.get();
        }
}
 
/**
* 测试线程
*
* @author leizhimin 2010-1-5 10:39:18
*/

public class TestThread extends Thread {
        private MyThreadLocal tlt = new MyThreadLocal();

        public TestThread(MyThreadLocal tlt) {
                this.tlt = tlt;
        }

        @Override
        public void run() {
                for (int i = 0; i < 3; i++) {
                        System.out.println(Thread.currentThread().getName() + "\t" + tlt.getNextNum());
                }
        }
}
 
/**
* ThreadLocal测试
*
* @author leizhimin 2010-1-5 10:43:48
*/

public class Test {
        public static void main(String[] args) {
                MyThreadLocal tlt = new MyThreadLocal();
                Thread t1 = new TestThread(tlt);
                Thread t2 = new TestThread(tlt);
                Thread t3 = new TestThread(tlt);
                Thread t4 = new TestThread(tlt);
                t1.start();
                t2.start();
                t3.start();
                t4.start();

        }
}
 
可以看出,三个线程各自独立编号,互不影响:
Thread-0  1
Thread-1  1
Thread-0  2
Thread-1  2
Thread-0  3
Thread-1  3
Thread-2  1
Thread-3  1
Thread-2  2
Thread-3  2
Thread-2  3
Thread-3  3

Process finished with exit code 0
 
tlt对象是一个,废话tl对象也是一个,因为组合关系是一对一的。但是tl对象内部的Map随着线程的增多,会创建很多Integer对象。只是Integer和int已经通用了。所以感觉不到Integer的对象属性。
 
二、不用ThreadLocal
 
假如不用ThreadLocal,只需要将MyThreadLocal类重新定义为:
/**
* 使用了ThreadLocal的类
*
* @author leizhimin 2010-1-5 10:35:27
*/

public class MyThreadLocal {
        private Integer t1 = 0;
        public Integer getNextNum(){
                return t1=t1+1;
        }



//        //定义了一个ThreadLocal变量,用来保存int或Integer数据
//        private ThreadLocal<Integer> tl = new ThreadLocal<Integer>() {
//                @Override
//                protected Integer initialValue() {
//                        return 0;
//                }
//        };
//
//        public Integer getNextNum() {
//                //将tl的值获取后加1,并更新设置t1的值
//                tl.set(tl.get() + 1);
//                return tl.get();
//        }
}
 
然后运行测试:
Thread-2  1
Thread-2  2
Thread-1  4
Thread-1  6
Thread-3  3
Thread-3  9
Thread-3  10
Thread-1  8
Thread-0  7
Thread-0  11
Thread-0  12
Thread-2  5

Process finished with exit code 0
 
从这里可以看出,四个线程共享了tlt变量,结果每个线程都直接修改tlt的属性。
 
三、自己实现个ThreadLocal
 
package com.lavasoft.test2;

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

/**
* 使用了ThreadLocal的类
*
* @author leizhimin 2010-1-5 10:35:27
*/

public class MyThreadLocal {

        //定义了一个ThreadLocal变量,用来保存int或Integer数据
        private com.lavasoft.test2.ThreadLocal<Integer> tl = new com.lavasoft.test2.ThreadLocal<Integer>() {
                @Override
                protected Integer initialValue() {
                        return 0;
                }
        };

        public Integer getNextNum() {
                //将tl的值获取后加1,并更新设置t1的值
                tl.set(tl.get() + 1);
                return tl.get();
        }
}

class ThreadLocal<T> {
        private Map<Thread, T> map = Collections.synchronizedMap(new HashMap<Thread, T>());

        public ThreadLocal() {
        }

        protected T initialValue() {
                return null;
        }

        public T get() {
                Thread t = Thread.currentThread();
                T obj = map.get(t);
                if (obj == null && !map.containsKey(t)) {
                        obj = initialValue();
                        map.put(t, obj);
                }
                return obj;
        }

        public void set(T value) {
                map.put(Thread.currentThread(), value);
        }

        public void remove() {
                map.remove(Thread.currentThread());
        }
}
 
运行测试:
Thread-0  1
Thread-0  2
Thread-0  3
Thread-2  1
Thread-2  2
Thread-3  1
Thread-2  3
Thread-3  2
Thread-1  1
Thread-3  3
Thread-1  2
Thread-1  3

Process finished with exit code 0
 
很意外,这个山寨版的ThreadLocal也同样运行很好,实现了JavaAPI中ThreadLocal的功能。
 
四、透过现象看本质
 
其实从程序角度看,tlt变量的确是一个,毫无疑问的。但是为什么打印出来的数字就互不影响呢?
是因为使用了Integer吗?-----不是。
原因是:protected T initialValue()和get(),因为每个线程在调用get()时候,发现Map中不存在就创建。调用它的时候,就创建了一个新变量,类型为T。每次都新建,当然各用个的互不影响了。
为了看清本质,将Integer换掉,重写部分类:
 
package com.lavasoft.test2;

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

/**
* 使用了ThreadLocal的类
*
* @author leizhimin 2010-1-5 10:35:27
*/

public class MyThreadLocal {

        //定义了一个ThreadLocal变量,用来保存int或Integer数据
        //        private ThreadLocal<Bean> tl = new ThreadLocal<Bean>() {
        private com.lavasoft.test2.ThreadLocal<Bean> tl = new com.lavasoft.test2.ThreadLocal<Bean>() {
                @Override
                protected Bean initialValue() {
                        return new Bean();
                }
        };

        @Override
        public String toString() {
                return "MyThreadLocal{" +
                                "tl=" + tl +
                                '}';
        }

        public Bean getBean() {
                return tl.get();
        }

}

class ThreadLocal<T> {
        private Map<Thread, T> map = Collections.synchronizedMap(new HashMap<Thread, T>());

        public ThreadLocal() {
        }

        protected T initialValue() {
                return null;
        }

        public T get() {
                Thread t = Thread.currentThread();
                T obj = map.get(t);
                if (obj == null && !map.containsKey(t)) {
                        obj = initialValue();
                        map.put(t, obj);
                }
                return obj;
        }

        public void set(T value) {
                map.put(Thread.currentThread(), value);
        }

        public void remove() {
                map.remove(Thread.currentThread());
        }
}
 
package com.lavasoft.test2;

/**
* 测试Bean
*
* @author leizhimin 2010-1-5 14:18:26
*/

public class Bean {
        private String id = "0";
        private String name = "none";

        public Bean() {
        }

        public Bean(String id, String name) {
                this.id = id;
                this.name = name;
        }

        public String getId() {
                return id;
        }

        public void setId(String id) {
                this.id = id;
        }

        public String getName() {
                return name;
        }

        public void setName(String name) {
                this.name = name;
        }

        public String showinfo() {
                return "Bean{" +
                                "id='" + id + '\'' +
                                ", name='" + name + '\'' +
                                '}';
        }
}
 
package com.lavasoft.test2;

/**
* 测试线程
*
* @author leizhimin 2010-1-5 10:39:18
*/

public class TestThread extends Thread {
        private MyThreadLocal tlt = new MyThreadLocal();

        public TestThread(MyThreadLocal tlt) {
                this.tlt = tlt;
        }

        @Override
        public void run() {
                System.out.println(">>>>>:" + tlt);
                for (int i = 0; i < 3; i++) {
                        System.out.println(Thread.currentThread().getName() + "\t" +tlt.getBean()+"\t"+tlt.getBean().showinfo());
                }
        }
}
 
然后运行测试:
>>>>>:MyThreadLocal{tl=com.lavasoft.test2.MyThreadLocal$1@1de3f2d}
>>>>>:MyThreadLocal{tl=com.lavasoft.test2.MyThreadLocal$1@1de3f2d}
>>>>>:MyThreadLocal{tl=com.lavasoft.test2.MyThreadLocal$1@1de3f2d}
>>>>>:MyThreadLocal{tl=com.lavasoft.test2.MyThreadLocal$1@1de3f2d}
Thread-1  com.lavasoft.test2.Bean@291aff  Bean{id='0', name='none'}
Thread-2  com.lavasoft.test2.Bean@fe64b9  Bean{id='0', name='none'}
Thread-3  com.lavasoft.test2.Bean@186db54  Bean{id='0', name='none'}
Thread-2  com.lavasoft.test2.Bean@fe64b9  Bean{id='0', name='none'}
Thread-2  com.lavasoft.test2.Bean@fe64b9  Bean{id='0', name='none'}
Thread-0  com.lavasoft.test2.Bean@291aff  Bean{id='0', name='none'}
Thread-3  com.lavasoft.test2.Bean@186db54  Bean{id='0', name='none'}
Thread-3  com.lavasoft.test2.Bean@186db54  Bean{id='0', name='none'}
Thread-1  com.lavasoft.test2.Bean@291aff  Bean{id='0', name='none'}
Thread-0  com.lavasoft.test2.Bean@291aff  Bean{id='0', name='none'}
Thread-0  com.lavasoft.test2.Bean@291aff  Bean{id='0', name='none'}
Thread-1  com.lavasoft.test2.Bean@291aff  Bean{id='0', name='none'}

Process finished with exit code 0
 
从打印结果很清楚的看到,MyThreadLocal的tlt对象的确是一个,tlt对象里的ThreadLocal的tl对象也是一个,但是,将t1t给每个线程用的时候,线程会重新创建Bean对象加入到ThreadLocal的Map中去使用。