理解ThreadLocal的含义

 

从定义上说ThreadLocal表示的是当前线程的本地变量

理解一下就是表示当前线程的一个变量，这个变量和其他线程完全隔离，只能在当前线程内访问

源码如下

public T get() {
        Thread t = Thread.currentThread(); // <1>
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();
    }

ThreadLocalMap getMap(Thread t) {
    return t.threadLocals;
}

public void set(T value) {
        Thread t = Thread.currentThread(); // <2>
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

重点在在<1>  <2>处，获取和设置都使用Thread.currentThread来获取当前线程，是当前线程作为一个KEY值去获取 VALUE

 

测试代码：

package com.my.pro.dao;

import com.my.pro.util.ProThreadLocal;


public class ProDAO {

    public void addDaoValue(String value) {
        String curThreadParam = ProThreadLocal.instance().get();
        System.out.println("DAO curThreadParam : " + curThreadParam);
        System.out.println("DAO curThreadName : " + Thread.currentThread().getName());
    }
}

package com.my.pro.service;

import com.my.pro.dao.ProDAO;
import com.my.pro.util.ProThreadLocal;


public class ProService {
    public void addSerValueSer(String value) {
        String curThreadParam = ProThreadLocal.instance().get();
        System.out.println("Service curThreadParam : " + curThreadParam);
        System.out.println("Service curThreadName : " + Thread.currentThread().getName());
        new ProDAO().addDaoValue(value);
    }
}


package com.my.pro.util;

public class ProThreadLocal {

    public static ThreadLocal<String> curThreadLocalParams = new ThreadLocal<String>();

    private ProThreadLocal() {
    }

    private static class ProThreadLocalHolder {
        private static ProThreadLocal INSTANCE = new ProThreadLocal();
    }

    public static ProThreadLocal instance() {
        return ProThreadLocalHolder.INSTANCE;
    }

    public String get() {
        return curThreadLocalParams.get();
    }

    public void set(String prarm) {
        curThreadLocalParams.set(prarm);
    }
}

执行

package com.my.pro;

import com.my.pro.service.ProService;
import com.my.pro.util.ProThreadLocal;


public class Main {

    public static void main(String args[]) {


        Thread threadA = new Thread(new Runnable() {
            @Override
            public void run() {
                ProThreadLocal.instance().set("Param-A");
                new ProService().addSerValueSer("-1");
            }
        });
        threadA.setName("Thread-a");
        threadA.start();

        Thread threadB = new Thread(new Runnable() {
            @Override
            public void run() {
                ProThreadLocal.instance().set("Param-B");
                new ProService().addSerValueSer("-2");
            }
        });
        threadB.setName("Thread-b");
        threadB.start();

    }
}

ThreadLocal类获取提供了get() set()方法来获取值

多次运行结果，Thread-a 和Thread-b中运行的代码以及调用链中的代码都只能获取自己线程的值。

 

ThreadLocal的使用：

在使用自定义的注解对切面方法进行处理的过程中，需要把返回值传给方法被注解的方法，就可以使用ThreadLocal存储返回值在方法中直接获取。

下次再写个实例。