被面试官吊打系列之Java基础 之 ThreadLocal 源码分析

其实 ThreadLocal 命名不太好，应当改名叫做 thread-local variable，即线程本地变量。一个ThreadLocal实例就是一个线程本地变量。它的特点是就是 任何时候同一个线程可以通过这个 ThreadLocal实例变量 访问到绑定的值 （其实有点绕）；需要注意的是 init方法，它默认什么都不做，通常我们需要覆写它，当然，这个完全是按照需要来定的。 每次线程执行get的时候，如果发现其内部的map还未创建，那么就创建，并且调用init初始化；

———— 更新，这个说法是不太准确的，虽然 ThreadLocalMap是ThreadLocal的静态内部类，但是并不会对每个ThreadLocal 创建一个ThreadLocalMap， 而是整个jvm所有ThreadLocal 共用一个ThreadLocalMap~！

浅尝辄止容易，深入浅出就很难。很多时候只要面试官多问几句，自己就会陷入懵逼状态了； 不是不懂，只是总有一些地方说不清道不明~.. 没有十分的清清楚楚明明白白，不敢乱说，干脆就糊弄一下；但是就会被面试官发现马脚。所以还是需要认真的研读一些源码。

源码大致分析
1 我之前以为是Thread 对ThreadLocal提供的支持， 其实不是的，跟Thread 几乎没什么关系。其实 ThreadLocal内部有一个静态来 ThreadLocalMap， 这个可谓是重中之重； 理解了它就理解了ThreadLocal。 ThreadLocalMap首先它是静态的，意味着它整个jvm中只需要创建一次！~ ThreadLocalMap顾名思义 是ThreadLocal的Map，但是它并不是内部有一个map字段之类的，也不是它继承了HashMap之类的已有的Map实现，它是自己重新实现了map的逻辑； 基本上就是内部一个数组Entry[] table，key的类型固定是ThreadLocal，value当然是不固定的，是静态内部类ThreadLocalMap.Entry, 这个Entry 非常的特别，继承了WeakReference<ThreadLocal<?>>； 然后扩容的时候，也是每次*2； 和HashMap的差别还是蛮大的；先到这里，没有时间研究透彻..xxx（这个大概也是不直接使用HaspMap的原因吧）
（WeakReference 确保了线程终止之后，对应的线程变量即ThreadLocal实例能够被回收~！）

2 使用的时候， 我们直接调用 ThreadLocal实例对象（比如tl）的的get方法，如果是整个jvm中第一次，那么创建ThreadLocalMap，然后 tl 调用 getEntry （不知道为什么没有get方法，而是提供了一个 getEntry方法），参数是自身，也就是ThreadLocal实例对象tl， 这个有点奇怪，不过也是可以理解的，那么getEntry方法内部计算 tl 的hash值等，找到它在map中的数组中对应的槽位等，然后返回； 如果调用tl的set方法，也是类似的，像hashMap的内部一样， 有一个通过key计算hash值的过程。 remove也是一样， 需要把tl对应的entry 删除掉。

换句话说，每一个 ThreadLocal实例对象 其实是对应了 ThreadLocalMap实例对象的一个 Entry， 每次ThreadLocal实例对象的操作其实是在操作这个Entry~！

 

使用场景呢？
就是需要在线程上绑定一些变量的时候，非常有用； 如果一个线程需要执行很多很多的操作，可能跨域了很多个方法、类级别的调用，同时可能耗时也比较长， 然后； 有点类似全局变量，但它又是线程安全的。
具体来说，当很多线程需要多次使用同一个对象，并且需要该对象具有相同初始化值的时候最适合使用ThreadLocal。最常见的ThreadLocal使用场景为 用来解决 数据库连接、Session管理等。

 

ThreadLocal VS 多线程锁

乍看起来和多线程的一些锁有点类似，但是差别非常大，使用场景完全不同。可以说，ThreadLocal提供了一个单线程的环境，从而它是线程安全的变量，它避免了多线程的线程安全问题；get、set、remove 的时候 当然是不需要考虑并发问题的， 因为都只将在当前线程中调用； ThreadLocal为每个线程都提供了变量的副本，使得每个线程在某一时间訪问到的并非同一个对象，这样就隔离了多个线程对数据的数据共享

package java.lang;
import java.lang.ref.*;
import java.util.Objects;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.Supplier;

/**
 * This class provides thread-local variables.  These variables differ from
 * their normal counterparts in that each thread that accesses one (via its
 * {@code get} or {@code set} method) has its own, independently initialized
 * copy of the variable.  {@code ThreadLocal} instances are typically private
 * static fields in classes that wish to associate state with a thread (e.g.,
 * a user ID or Transaction ID).
 *
 * <p>For example, the class below generates unique identifiers local to each
 * thread.
 * A thread's id is assigned the first time it invokes {@code ThreadId.get()}
 * and remains unchanged on subsequent calls.
 
 这个类提供了线程本地变量。 这些变量与普通的对应变量不同的是，每个访问变量的线程（通过其{@code get}或{@code set}方法）都有自己的、独立的初始化变量副本。 {@code ThreadLocal}实例通常是类中希望将状态与线程关联的私有静态字段（例如，用户ID或事务ID）。<p>例如，下面的类会生成每个线程的本地唯一标识符。线程的id在第一次调用{@code ThreadId.get()}时被分配，并在后续调用时保持不变。
 
 * <pre>
 * import java.util.concurrent.atomic.AtomicInteger;
 *
 * public class ThreadId {
 *     // Atomic integer containing the next thread ID to be assigned
 *     private static final AtomicInteger nextId = new AtomicInteger(0);
 *
 *     // Thread local variable containing each thread's ID
 *     private static final ThreadLocal&lt;Integer&gt; threadId =
 *         new ThreadLocal&lt;Integer&gt;() {
 *             &#64;Override protected Integer initialValue() {
 *                 return nextId.getAndIncrement();
 *         }
 *     };
 *
 *     // Returns the current thread's unique ID, assigning it if necessary
 *     public static int get() {
 *         return threadId.get();
 *     }
 * }
 * </pre>
 * <p>Each thread holds an implicit reference to its copy of a thread-local
 * variable as long as the thread is alive and the {@code ThreadLocal}
 * instance is accessible; after a thread goes away, all of its copies of
 * thread-local instances are subject to garbage collection (unless other
 * references to these copies exist).
 *
 * @author  Josh Bloch and Doug Lea
 * @since   1.2
 */
public class ThreadLocal<T> {
    /**
     * ThreadLocals rely on per-thread linear-probe hash maps attached
     * to each thread (Thread.threadLocals and
     * inheritableThreadLocals).  The ThreadLocal objects act as keys,
     * searched via threadLocalHashCode.  This is a custom hash code
     * (useful only within ThreadLocalMaps) that eliminates collisions
     * in the common case where consecutively constructed ThreadLocals
     * are used by the same threads, while remaining well-behaved in
     * less common cases.
     */
    private final int threadLocalHashCode = nextHashCode();

    /**
     * The next hash code to be given out. Updated atomically. Starts at
     * zero.
     */
    private static AtomicInteger nextHashCode =
        new AtomicInteger();

    /**
     * The difference between successively generated hash codes - turns
     * implicit sequential thread-local IDs into near-optimally spread
     * multiplicative hash values for power-of-two-sized tables.
     */
    private static final int HASH_INCREMENT = 0x61c88647;

    /**
     * Returns the next hash code.
     */
    private static int nextHashCode() {
        return nextHashCode.getAndAdd(HASH_INCREMENT);
    }

    /**
     * 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.
     
     返回当前线程本地变量的 "初始值"。 这个方法将在线程第一次使用{@link #get}方法访问该变量时被调用，除非该线程之前调用了{@link #set}方法，在这种情况下，{@code initialValue}方法不会被调用。 通常情况下，这个方法在每个线程中最多调用一次，但是在后续调用{@link #remove}和{@link #get}的情况下，这个方法可能会被再次调用。  <p>这个实现只需返回{@code null}；如果程序员希望线程本地变量的初始值不是{@code null}，那么必须子类化{@code ThreadLocal}，并重载这个方法。 通常情况下，会使用一个匿名的内部类。
     
     *
     * @return the initial value for this thread-local
     */
    protected T initialValue() {
        return null;
    }

    /**
     * Creates a thread local variable. The initial value of the variable is
     * determined by invoking the {@code get} method on the {@code Supplier}.
     
 创建一个线程的本地变量，变量的初始值由调用{@code Supplier}上的{@code get}方法决定。变量的初始值由调用{@code get}方法决定。
     
     *
     * @param <S> the type of the thread local's value
     * @param supplier the supplier to be used to determine the initial value
     * @return a new thread local variable
     * @throws NullPointerException if the specified supplier is null
     * @since 1.8
     */
     SuppliedThreadLocal 也是内部的静态final类：
    public static <S> ThreadLocal<S> withInitial(Supplier<? extends S> supplier) {
        return new SuppliedThreadLocal<>(supplier);
    }

    /**
     * Creates a thread local variable.
     * @see #withInitial(java.util.function.Supplier)
     */
    public ThreadLocal() {
    }

    /**
     * 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.
     返回当前线程本地变量的副本中的值。 如果这个变量对当前线程没有值，那么它首先被初始化为调用{@link #initialValue}方法返回的值。
     
     *
     * @return the current thread's value of this thread-local    该线程的当前线程的本地值
     */
    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;
    }

    /**
     * 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.
     将当前线程的本地变量的副本设置为指定的值。 大多数子类不需要覆盖这个方法，只需要依靠{@link #initialValue}方法来设置线程本地变量的值。
     *
     * @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.
     
   删除当前线程对这个线程本地变量的值。 如果这个线程本地变量随后被当前线程{@linkplain #get read}，它的值将通过调用它的{@link #initialValue}方法重新初始化，除非它的值被当前线程{@linkplain #set set}。 这可能导致在当前线程中多次调用{@code initialValue}方法。
   
     *
     * @since 1.5
     */
     public void remove() {
         ThreadLocalMap m = getMap(Thread.currentThread());
         if (m != null)
             m.remove(this);
     }

    /**
     * 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;
    }

    /**
     * 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);
    }

    /**
     * Factory method to create map of inherited thread locals.
     * Designed to be called only from Thread constructor.
     *
     * @param  parentMap the map associated with parent thread
     * @return a map containing the parent's inheritable bindings
     */
    static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
        return new ThreadLocalMap(parentMap);
    }

    /**
     * Method childValue is visibly defined in subclass
     * InheritableThreadLocal, but is internally defined here for the
     * sake of providing createInheritedMap factory method without
     * needing to subclass the map class in InheritableThreadLocal.
     * This technique is preferable to the alternative of embedding
     * instanceof tests in methods.
     */
    T childValue(T parentValue) {
        throw new UnsupportedOperationException();
    }

    /**
     * An extension of ThreadLocal that obtains its initial value from
     * the specified {@code Supplier}.
     */
    static final class SuppliedThreadLocal<T> extends ThreadLocal<T> {

        private final Supplier<? extends T> supplier;

        SuppliedThreadLocal(Supplier<? extends T> supplier) {
            this.supplier = Objects.requireNonNull(supplier);
        }

        @Override
        protected T initialValue() {
            return supplier.get();
        }
    }

    /**
     * ThreadLocalMap is a customized hash map suitable only for
     * maintaining thread local values. No operations are exported
     * outside of the ThreadLocal class. The class is package private to
     * allow declaration of fields in class Thread.  To help deal with
     * very large and long-lived usages, the hash table entries use
     * WeakReferences for keys. However, since reference queues are not
     * used, stale entries are guaranteed to be removed only when
     * the table starts running out of space.
     
     
ThreadLocalMap是一个自定义的哈希map，只适用于维护线程本地值。在ThreadLocal类之外，没有任何操作被导出。该类是封装私有的，允许在类Thread中声明字段。 为了帮助处理非常大的和长时间的使用，哈希表条目使用WeakReferences作为键值。但是，由于不使用引用队列，所以只有当表开始用完空间时，陈旧的条目才会被保证删除。

     */
    static class ThreadLocalMap {    静态内部类 

        /**
         * 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.
         
          这个哈希map中的入口条目扩展了WeakReference，使用它的主ref字段作为键（总是一个ThreadLocal对象）。 注意，空键（即 entry.get() == null）意味着该键不再被引用，因此该条目可以从表中删除。 这样的条目在下面的代码中被称为 "陈旧的条目"。
         */
        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;

        /**
         * The number of entries in the table.
         */
        private int size = 0;

        /**
         * The next size value at which to resize.
         */
        private int threshold; // Default to 0

        /**
         * Set the resize threshold to maintain at worst a 2/3 load factor.
         */
        private void setThreshold(int len) {
            threshold = len * 2 / 3;
        }

        /**
         * Increment i modulo len.
         */
        private static int nextIndex(int i, int len) {
            return ((i + 1 < len) ? i + 1 : 0);
        }

        /**
         * Decrement i modulo len.
         */
        private static int prevIndex(int i, int len) {
            return ((i - 1 >= 0) ? i - 1 : len - 1);
        }

        /**
         * Construct a new map initially containing (firstKey, firstValue).
         * ThreadLocalMaps are constructed lazily, so we only create
         * one when we have at least one entry to put in it.
         */
        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);
        }

        /**
         * Get the entry associated with key.  This method
         * itself handles only the fast path: a direct hit of existing
         * key. It otherwise relays to getEntryAfterMiss.  This is
         * designed to maximize performance for direct hits, in part
         * by making this method readily inlinable.
         *
         * @param  key the thread local object
         * @return the entry associated with key, or null if no such
         */
        private Entry getEntry(ThreadLocal<?> key) {
            int i = key.threadLocalHashCode & (table.length - 1);
            Entry e = table[i];
            if (e != null && e.get() == key)
                return e;
            else
                return getEntryAfterMiss(key, i, e);
        }

        ....
}

 

参考：
https://blog.csdn.net/ityouknow/article/details/90709371
https://blog.csdn.net/liuhaiabc/article/details/78077529
https://www.cnblogs.com/yxysuanfa/p/7125761.html