深入理解--Threadlocal原理，及可能导致内存泄露

àì夳堔傛蜴生んèń 2023-01-10 10:25 224阅读 0赞

一.原理

（1）什么是ThreadLocal变量

ThreadLoal 变量，线程局部变量，ThreadLocal 提供了线程本地的实例，它与普通变量的区别在于，每个使用该变量的线程都会初始化一个完全独立的实例副本。当一个线程结束时，它所使用的所有 ThreadLocal 相对的实例副本都可被回收。

总的来说，ThreadLocal 适用于每个线程需要自己独立的实例且该实例需要在多个方法中被使用，也即变量在线程间隔离而在方法或类间共享的场景。

（2）ThreadLocal内部数据结构

首先 ThreadLocal 是一个泛型类，保证可以接受任何类型的对象。ThreadLocal 内部维护了一个 Map ，这个 Map 不是直接使用的 HashMap ，而是 ThreadLocal 实现的一个叫做 ThreadLocalMap 的静态内部类。而我们使用的 get()、set() 方法其实都是调用了这个ThreadLocalMap类对应的 get()、set() 方法。

部分源码如下：

//内部map

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;
            }
        }
    
        /**
         * The initial capacity -- MUST be a power of two.
         */
        private static final int INITIAL_CAPACITY = 16;
    
        /**
         * The table, resized as necessary.
         * table.length MUST always be a power of two.
         */
        private Entry[] table;

//get 方法，特别注意：每次get发现map为空，都会调用setInitialValue方法，返回一个value为null

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();
    }
    
    /**
     * 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;
    }
    
    /** * 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; }
    
    /**
     * 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);
    }

/**
     * 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);
     }

二.可能内存泄露原因

![d23a842f29a6d5803f9ac5a2399e0e89.png][]

在threadlocal的生命周期中,都存在这些引用. 如上图: 实线代表强引用,虚线代表弱引用.

原因： ThreadLocal内部为每个线程中都维护一个类似于Map，类型弱引用的ThreadLocalMap。Map中的key为一个threadlocal实例. 这个Map使用了弱引用，只是针对key， 当把threadlocal实例置为null以后，没有任何强引用指向threadlocal实例,所以threadlocal将会被gc回收.

但是，map中的value却不能回收，因为存在一条从current thread连接过来的强引用，当释放掉threadlocal的强引用以后，map里面的value却没有被回收, 所以存在着内存泄露.。

解决办法：手动调用threadlocal的remove方法.

[d23a842f29a6d5803f9ac5a2399e0e89.png]: /images/20221119/cead3e3bbfd14c138b4c1ded067f67e0.png