浅谈ThreadLocal和InheritableThreadLocal，源码解析

ThreadLocal浅谈

首先有个概念可以提供大家参考：threadLocal只是操作thread类中threadLocalMap变量的一个工具，理解了这个对大家理解下边文章有好处

1，ThreadLocal的创建

一般创建一个ThreadLocal对象ThreadLocal threadLocal = new ThreadLocal<>()生成一个ThreadLocal对象，

2，ThreadLocal的set方法源码

当对该对象进行set操作的时候，参考源码如下：

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

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

流程为，获取当前执行线程 Thread t = Thread.currentThread();
getMap方法中获取当前线程的线程变量 t.threadLocals，
而threadLocals 是Thread类型的变量 ThreadLocal.ThreadLocalMap
ThreadLocalMap属于ThreadLocal中的内部类，是一个Entry数组，

第一次创建的ThreadLocal的ThreadLocal.ThreadLocalMap threadLocals = null;默认是null；

所以第一次的set方法会执行createMap(t, value);方法

void createMap(Thread t, T firstValue) {
    
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
    
            table = new Entry[INITIAL_CAPACITY];
            int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
            table[i] = new Entry(firstKey, firstValue);
            size = 1;
            setThreshold(INITIAL_CAPACITY);
        }

createMap是生成了一个ThreadLocalMap对象并赋值给当前线程变量threadLocals，
到这里ThreadLocal的set方法执行完毕。

3，ThreadLocal的get方法源码

public T get() {
    
        Thread t = Thread.currentThread();
        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();
    }

get源码是比较简单的，在获取到当前线程的ThreadLocalMap之后去获取ThreadLocalMap中entry中存储的值，
比较有意思的是这个setInitialValue()方法；

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;
    }

当生成threadLocal未进行set操作之前进行get操作，会执行这个setInitialValue（）方法，会创建一个key值为this，value为null的数据放入当前线程的threadLocalMap中

3，ThreadLocal的remove方法源码

remove源码更为简单，做了当前threadLocalMap的清空工作

public void remove() {
    
         ThreadLocalMap m = getMap(Thread.currentThread());
         if (m != null)
             m.remove(this);
     }

至此ThreadLocal的基本源码就阅读完成了

InheritableThreadLocal浅谈

InheritableThreadLocal为ThreadLocal的子类，复写了ThreadLocal的单个方法

public class InheritableThreadLocal<T> extends ThreadLocal<T> {
    
    /** * Computes the child's initial value for this inheritable thread-local * variable as a function of the parent's value at the time the child * thread is created. This method is called from within the parent * thread before the child is started. * <p> * This method merely returns its input argument, and should be overridden * if a different behavior is desired. * * @param parentValue the parent thread's value * @return the child thread's initial value */
    protected T childValue(T parentValue) {
    
        return parentValue;
    }

    /** * Get the map associated with a ThreadLocal. * * @param t the current thread */
    ThreadLocalMap getMap(Thread t) {
    
       return t.inheritableThreadLocals;
    }

    /** * Create the map associated with a ThreadLocal. * * @param t the current thread * @param firstValue value for the initial entry of the table. */
    void createMap(Thread t, T firstValue) {
    
        t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
    }
}

1，InheritableThreadLocal和ThreadLocal的区别和联系

InheritableThreadLocal可以实现父子线程变量的共享，而ThreadLocal变量不能在其子线程中获取，

2，InheritableThreadLocal是如何实现父字线程变量共享的

当我们创建一个子线程时，执行了Thread类型以下源码

 public Thread() {
    
        init(null, null, "Thread-" + nextThreadNum(), 0);
    }

private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize) {
    
        init(g, target, name, stackSize, null, true);
    }

private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize, AccessControlContext acc,
                      boolean inheritThreadLocals) {
    
    
        
		****省略的代码
        
        if (security == null || isCCLOverridden(parent.getClass()))
            this.contextClassLoader = parent.getContextClassLoader();
        else
            this.contextClassLoader = parent.contextClassLoader;
        this.inheritedAccessControlContext =
                acc != null ? acc : AccessController.getContext();
        this.target = target;
        setPriority(priority);
        if (inheritThreadLocals && parent.inheritableThreadLocals != null)
            this.inheritableThreadLocals =
                ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
        /* Stash the specified stack size in case the VM cares */
        this.stackSize = stackSize;

        /* Set thread ID */
        tid = nextThreadID();
    }

可以看到最后一个init方法中，也就是Thread类第418行到420的代码
有一个将当前线程inheritableThreadLocals的复制搬运动作，来完成了父子线程间的通讯

那么为什么inheritableThreadLocals会不为null呢，因为InheritableThreadLocal复写了ThreadLocal的getMap方法，返回了Thread变量中的inheritableThreadLocals实现初始化

ThreadLocal用法举例

1，例如做一些oa系统，可能会涉及在代码中获取当前登录人的信息，可以通过ThreadLocal去做一些当前登录人信息的记录

2，接口耗时的日志记录也可以记录用ThreadLocal做一些记录动作