代碼演示ThreadLocal原理

ThreadLocal

仿ThreadLocal源碼理解其原理；

1.定義自己的ThreadLocal實現

import java.util.HashMap;

public class MyThreadLocal<T>
{
    public class MyThreadLocalMap
    {
        private HashMap<String, T> map;

        private MyThreadLocalMap()
        {
            map = new HashMap<String, T>();
        }
    }

    public T initValue()
    {
        return null;
    }

    public void createMap(MyThread t)
    {
        t.setLocalMap(new MyThreadLocalMap());
    }

    public T setInitValue(MyThread t)
    {
        T value = initValue();
        MyThreadLocalMap map = t.getLocalMap();
        if (null == map)
        {
            createMap(t);
        }

        t.getLocalMap().map.put(String.valueOf(this.hashCode()), value);

        return value;
    }

    public void set(T value)
    {
        MyThread thread = (MyThread)Thread.currentThread();
        MyThreadLocalMap localMap = thread.getLocalMap();
        if (null == localMap)
        {
            createMap(thread);
        }

        thread.getLocalMap().map.put(String.valueOf(this.hashCode()), value);
    }

    public T get()
    {
        MyThread thread = (MyThread)Thread.currentThread();
        MyThreadLocalMap localMap = thread.getLocalMap();

        if (null != localMap)
        {
            return localMap.map.get(String.valueOf(this.hashCode()));
        }

        return setInitValue(thread);
    }

    public void remove()
    {
        MyThread thread = (MyThread)Thread.currentThread();
        MyThreadLocalMap localMap = thread.getLocalMap();

        if (null != localMap)
        {
            localMap.map.remove(String.valueOf(this.hashCode()));
        }
    }
}

2.繼承Thread實現自己的Thread

public class MyThread extends Thread
{
    private MyThreadLocal.MyThreadLocalMap localMap;

    public MyThread(Runnable runnable)
    {
        super(runnable);
    }

    public void setLocalMap(MyThreadLocal.MyThreadLocalMap map)
    {
        localMap = map;
    }

    public MyThreadLocal.MyThreadLocalMap getLocalMap()
    {
        return localMap;
    }
}

測試代碼：

public class Main
{
    public static void main(String[] args)
    {
        Runnable runnable = new Runnable() {
            public void run() {
                MyThreadLocal local = new MyThreadLocal() {
                    @Override
                    public Object initValue() {
                        return Thread.currentThread().getName();
                    }
                };

                System.out.println(local.get());
            }
        };

        MyThread thread1 = new MyThread(runnable);
        MyThread thread2 = new MyThread(runnable);

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

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

概述

ThreadLocal即線程變量（自己這麼叫的不是很準確），極大的方便線程獨享資源的管理，不需要再對資源進行管理避免了因多線程造成的問題；ThreadLocal將線程自己的數據封裝到內部，用ThreadLocal計算出hashcode爲鍵值（本文用的HashMap存儲，講明原理），通過內部類Entity實現存儲（哈希數組初始值16）；