深入了解ThreadLocal

Posted by shuyou on Friday, March 5, 2021

本文分析ThreadLocal的原理和使用

1.ThreadLocal简介

多线程访问共享变量时容易出现并发问题,为了保证线程安全,一般会给共享变量进行适当的加锁同步。如果不想加锁呢? ThreadLocal可以做到线程隔离,多个线程访问共享变量时,访问的是自己线程的变量。 ThreadLocal提供了线程本地变量,如果创建了一个ThreadLocal变量,那么访问这个变量的每个线程都会有这个变量的一个本地副本,当多线程操作这个变量时,实际操作的是自己本地内存的变量,从而避免线程安全的问题。

2.ThreadLocal使用

public class ThreadLocalDemo {

    static ThreadLocal<String> stringThreadLocal = new ThreadLocal<String>();


    public static void main(String[] args) {

        CountDownLatch countDownLatch = new CountDownLatch(10);
        for (int i = 0; i < 10; i++) {
            Thread thread = new Thread(new Runnable() {
                @Override
                public void run() {
                    stringThreadLocal.set(Thread.currentThread().getName());
                    System.out.println(stringThreadLocal.get());
                    countDownLatch.countDown();
                }
            },"i  am  thread  --"+i);
            thread.start();
        }

    }
}

运行结果 结果

3.ThreadLocal的原理

Thread类中有两个包访问变量,一个是threadLocals ,一个是inheritableThreadLocals,它们都是ThreadLocalMap类型的变量。 在这里插入图片描述 而ThreadLocalMap又是ThreadLocal的内部类。 默认情况下,每个线程的这两个变量都为null,只有当线程第一次调用ThreadLocal 的set 或者get方法时才会创建他们。 每个线程的本地变量是存在调用线程的threadLocals变量中的,ThreadLocal通过set方法把value放在调用线程的threadLocals变量中,通过get方法取出调用线程的threadLocals中的值。 Thread里面的threadLocals为何设计为map结构?因为每个线程可以关联多个ThreadLocal变量。

下面分析下 ThreadLocal 的set、get及remove方法 1.set

    public void set(T value) {
    	//获取当前调用线程
        Thread t = Thread.currentThread();
        //将当前线程作为key  去查对应的线程变量threadLocals
        ThreadLocalMap map = getMap(t);
        //当前线程的threadLocals不为null
        if (map != null)
        	//将当前ThreadLocal 对象作为key传入map
            map.set(this, value);
        else
        	//创建map
            createMap(t, value);
    }
    
    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }
    
    void createMap(Thread t, T firstValue) {
    	//当前线程的threadLocals  赋值   以当前ThreadLocal 对象作为key 创建的ThreadLocalMap
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

	//ThreadLocalMap的构造函数
    ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
    	//Entry为ThreadLocalMap的内部类  INITIAL_CAPACITY = 16
        table = new Entry[INITIAL_CAPACITY];
        //计算应该存放的位置 i  因INITIAL_CAPACITY = 16 相当于对 16 取余
        int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
        //存放到table[i]
        table[i] = new Entry(firstKey, firstValue);
        size = 1;
        setThreshold(INITIAL_CAPACITY);
    }

2.get

    public T get() {
    	//获取当前调用线程
        Thread t = Thread.currentThread();
        //将当前线程作为key  去查对应的线程变量threadLocals
        ThreadLocalMap map = getMap(t);
        if (map != null) {
        	//以当前ThreadLocal对象作为key 去取 map中的 entry
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();
    }

	private T setInitialValue() {
        T value = initialValue();
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
        return value;
    }

remove

    public void remove() {
        ThreadLocalMap m = getMap(Thread.currentThread());
        if (m != null)
            m.remove(this);
    }
    
	private void remove(ThreadLocal<?> key) {
			//拿到table数组
            Entry[] tab = table;
            int len = tab.length;
            //找到在数组中存放的位置 i
            int i = key.threadLocalHashCode & (len-1);
            for (Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) {
                //判断key是否相等
                if (e.get() == key) {
                	//清除
                    e.clear();
                    expungeStaleEntry(i);
                    return;
                }
            }
   	}

ThreadLocalMap内部类 Enrtry

        static class Entry extends WeakReference<ThreadLocal<?>> {
           
            Object value;

            Entry(ThreadLocal<?> k, Object v) {
                super(k);
                value = v;
            }
        }

Enrtry 继承自软引用,当对应的ThreadLocal对象为null时,此Entry对象会被JVM回收,避免出现内存泄漏。

4.ThreadLocal不支持继承性

public class TestThreadLocal {
    public static ThreadLocal<String> threadLocal = new ThreadLocal<String>();

    public static void main(String[] args) {
        threadLocal.set("hello");
        
        new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("sub thread:  " + threadLocal.get());
            }
        }).start();

        System.out.println("main:  "+ threadLocal.get());
    }
}

在这里插入图片描述 也就是说,同一个ThreadLocal变量在父线程中被设置值后,在子线程中是获取不到的。

5.InheritableThreadLocal类

利用InheritableThreadLocal类,子线程可以访问父线程中的本地变量。

//继承ThreadLocal类
public class InheritableThreadLocal<T> extends ThreadLocal<T> {

    protected T childValue(T parentValue) {
        return parentValue;
    }
	//返回当前线程的inheritableThreadLocals变量
    ThreadLocalMap getMap(Thread t) {
       return t.inheritableThreadLocals;
    }
    //初始化当前线程的inheritableThreadLocals变量
    void createMap(Thread t, T firstValue) {
        t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
    }
}

当子线程初始化时会判断父线程的inheritableThreadLocals变量是否为null,不为null 则会赋值给子线程inheritableThreadLocals变量 在这里插入图片描述

    static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
    	//这个构造函数 仅此方法createInheritedMap调用
        return new ThreadLocalMap(parentMap);
    }
    
    private ThreadLocalMap(ThreadLocalMap parentMap) {
        Entry[] parentTable = parentMap.table;
        int len = parentTable.length;
        setThreshold(len);
        table = new Entry[len];

        for (int j = 0; j < len; j++) {
            Entry e = parentTable[j];
            if (e != null) {
                @SuppressWarnings("unchecked")
                ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
                if (key != null) {
                	//这里调用InheritableThreadLocal类覆盖的 childValue方法
                    Object value = key.childValue(e.value);
                    Entry c = new Entry(key, value);
                    int h = key.threadLocalHashCode & (len - 1);
                    while (table[h] != null)
                        h = nextIndex(h, len);
                    table[h] = c;
                    size++;
                }
            }
        }
    }

改为InheritableThreadLocal 运行

public class TestInheritableThreadLocal {
    public static ThreadLocal<String> threadLocal = new InheritableThreadLocal<>();

    public static void main(String[] args) {
        threadLocal.set("hello");

        new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("sub thread:  " + threadLocal.get());
            }
        }).start();

        System.out.println("main:  "+ threadLocal.get());
    }
}

在这里插入图片描述

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