java ThreadLocal使用

Posted 2022-04-08 kingsonfu

1、源码分析

此处以JDK1.8版本分析

1.1 set方法

　　/**
     * Sets the current thread‘s copy of this thread-local variable
     * to the specified value.  Most subclasses will have no need to
     * override this method, relying solely on the @link #initialValue
     * method to set the values of thread-locals.
     *
     * @param value the value to be stored in the current thread‘s copy of
     *        this thread-local.
     */
    public void set(T value) 
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    

此处的ThreadLocalMap为ThreadLocal的一个内部类，用Entry来存储要设置的值。如下

　　　static class ThreadLocalMap 

        /**
         * The entries in this hash map extend WeakReference, using
         * its main ref field as the key (which is always a
         * ThreadLocal object).  Note that null keys (i.e. entry.get()
         * == null) mean that the key is no longer referenced, so the
         * entry can be expunged from table.  Such entries are referred to
         * as "stale entries" in the code that follows.
         */
        static class Entry extends WeakReference<ThreadLocal<?>> 
            /** The value associated with this ThreadLocal. */
            Object value;

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

看下getMap()方法，用于返回当前ThreadLocal维护的ThreadLocalMap对象，如下

    /**
     * Get the map associated with a ThreadLocal. Overridden in
     * InheritableThreadLocal.
     *
     * @param  t the current thread
     * @return the map
     */
    ThreadLocalMap getMap(Thread t) 
        return t.threadLocals;
    

此处的t.threadLocals为当前Thread维护的ThreadLocalMap对象

　　/* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;

再看下createMap()方法，用于为当前Thread没有ThreadLocalMap时新建一个ThreadLocalMap，如下

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

从源码中可以看出set方法过程：获取当前线程，并作为句柄去获取ThreadLocalMap，如果ThreadLocalMap存在，则以当前ThreadLocal对象为key，传入的对象为value进行存储。否则新建一个和当前线程相关联的ThreadLocalMap来存储。

1.2 get方法

　　/**
     * Returns the value in the current thread‘s copy of this
     * thread-local variable.  If the variable has no value for the
     * current thread, it is first initialized to the value returned
     * by an invocation of the @link #initialValue method.
     *
     * @return the current thread‘s value of this thread-local
     */
    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();
    

从源码可以看出也是以当前线程为句柄获取ThreadLocalMap对象，如果存在则返回当前线程的ThreadLocal为key对应的值，否则设置初始值setInitialValue

    /**
     * Variant of set() to establish initialValue. Used instead
     * of set() in case user has overridden the set() method.
     *
     * @return the initial value
     */
    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;
    

其中initialValue返回null

/**
     * Returns the current thread‘s "initial value" for this
     * thread-local variable.  This method will be invoked the first
     * time a thread accesses the variable with the @link #get
     * method, unless the thread previously invoked the @link #set
     * method, in which case the @code initialValue method will not
     * be invoked for the thread.  Normally, this method is invoked at
     * most once per thread, but it may be invoked again in case of
     * subsequent invocations of @link #remove followed by @link #get.
     *
     * <p>This implementation simply returns @code null; if the
     * programmer desires thread-local variables to have an initial
     * value other than @code null, @code ThreadLocal must be
     * subclassed, and this method overridden.  Typically, an
     * anonymous inner class will be used.
     *
     * @return the initial value for this thread-local
     */
    protected T initialValue() 
        return null;
    

1.3 remove()方法

    /**
     * Removes the current thread‘s value for this thread-local
     * variable.  If this thread-local variable is subsequently
     * @linkplain #get read by the current thread, its value will be
     * reinitialized by invoking its @link #initialValue method,
     * unless its value is @linkplain #set set by the current thread
     * in the interim.  This may result in multiple invocations of the
     * @code initialValue method in the current thread.
     *
     * @since 1.5
     */
     public void remove() 
         ThreadLocalMap m = getMap(Thread.currentThread());
         if (m != null)
             m.remove(this);
     

从源码可以很容易看出remove的作用过程，不做详细分析。为了避免内存泄漏问题，尽量在当前线程处理完之后手动进行remove操作。

2、总结

从上面的源码可以看出ThreadLocal设置的值和线程相关，可以理解为是线程的私有的访问区域，其他线程无法访问，实现了“数据隔离”效果。和解决并发问题和共享变量无关

每个Thread都维护者一个ThreadLocalMap的引用。