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<Integer> threadId = * new ThreadLocal<Integer>() { * @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