一、Handler机制与ThreadLocal
在Handler机制的时候，我们会接触到Looper中的一个很重要的类：ThreadLocal，ThreadLocal并不是线程，它的作用是可以在每个线程中存储数据。
Handler在创建的时候就会获取当前线程的Looper来构造消息循环系统，获取的方式就是通过ThreadLocal。ThreadLocal可以在不同的线程中互补干扰的存储并提供数据，Handler就是通过ThreadLocal可以轻松获取每个线程的Looper。当然需要注意的是，线程是默认没有Looper的，如果需要使用Handler就必须为线程创建Looper。
大家经常提到的主线程，也叫UI线程，它就是ActivityThread，ActivityThread被创建时就会初始化Looper，这也是在主线程中默认可以使用Handler的原因。
二、ThreadLocal 基本使用
ThreadLocal是一个线程内部的数据存储类，通过它可以在指定的线程中存储数据，数据存储以后，只有指定线程中可以获取到存储的数据，对于其它线程来说无法获取到数据。一般开发情景中我们用不到ThreadLocal，但是在一些非常复杂或者特殊的情景中， 通过ThreadLocal可以轻松地实现一些看起来很复杂的功能。
在Android的系统机制中，以下源码中使用到了ThreadLocal：
- Looper
- ActivityThread
- ActivityManagerService
具体到ThreadLocal的使用场景，一般可以概况为，当某些数据是以线程为作用域并且不同线程具有不同的数据副本的时候，就可以考虑采用ThreadLocal。
下面是使用ThreadLocal的最简单的例子：
public class ThreadLocalTest { static ThreadLocal threadLocal = new ThreadLocal(); public static void main(String[] args) { threadLocal.set(true); System.out.println("Main ThreadLocal Value = " + threadLocal.get()); new Thread(new Runnable() { @Override public void run() { threadLocal.set(false); System.out.println("ThreadLocal_1 Value = " + threadLocal.get()); } }).start(); new Thread(new Runnable() { @Override public void run() { System.out.println("ThreadLocal_2 Value = " + threadLocal.get()); } }).start(); } }
输出的结果如下：
Main ThreadLocal Value = true ThreadLocal_1 Value = false ThreadLocal_2 Value = null
可以看出，虽然在不同线程中访问的是同一个ThreadLocal对象，但是它们通过ThreadLocal来获取到的值却是不一样的，这就是ThreadLocal的奇妙之处。
三、ThreadLocal源码
/** * 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. * <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. * * @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}. * * @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 */ 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. * * @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(); } /** * 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. * * @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); } /** * 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. */ 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; /** * 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); } /** * Construct a new map including all Inheritable ThreadLocals * from given parent map. Called only by createInheritedMap. * * @param parentMap the map associated with parent thread. */ private ThreadLocalMap(ThreadLocalMap parentMap) { Entry[] parentTable = parentMap.table; int len = parentTable.length; setThreshold(len); table = new Entry[len]; for (int j = 0; j < len; j++) { Entry e = parentTable[j]; if (e != null) { @SuppressWarnings("unchecked") ThreadLocal<Object> key = (ThreadLocal<Object>) e.get(); if (key != null) { Object value = key.childValue(e.value); Entry c = new Entry(key, value); int h = key.threadLocalHashCode & (len - 1); while (table[h] != null) h = nextIndex(h, len); table[h] = c; size++; } } } } /** * 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); } /** * Version of getEntry method for use when key is not found in * its direct hash slot. * * @param key the thread local object * @param i the table index for key's hash code * @param e the entry at table[i] * @return the entry associated with key, or null if no such */ private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) { Entry[] tab = table; int len = tab.length; while (e != null) { ThreadLocal<?> k = e.get(); if (k == key) return e; if (k == null) expungeStaleEntry(i); else i = nextIndex(i, len); e = tab[i]; } return null; } /** * Set the value associated with key. * * @param key the thread local object * @param value the value to be set */ private void set(ThreadLocal<?> key, Object value) { // We don't use a fast path as with get() because it is at // least as common to use set() to create new entries as // it is to replace existing ones, in which case, a fast // path would fail more often than not. Entry[] tab = table; int len = tab.length; int i = key.threadLocalHashCode & (len-1); for (Entry e = tab[i]; e != null; e = tab[i = nextIndex(i, len)]) { ThreadLocal<?> k = e.get(); if (k == key) { e.value = value; return; } if (k == null) { replaceStaleEntry(key, value, i); return; } } tab[i] = new Entry(key, value); int sz = ++size; if (!cleanSomeSlots(i, sz) && sz >= threshold) rehash(); } /** * Remove the entry for key. */ private void remove(ThreadLocal<?> key) { Entry[] tab = table; int len = tab.length; int i = key.threadLocalHashCode & (len-1); for (Entry e = tab[i]; e != null; e = tab[i = nextIndex(i, len)]) { if (e.get() == key) { e.clear(); expungeStaleEntry(i); return; } } } /** * Replace a stale entry encountered during a set operation * with an entry for the specified key. The value passed in * the value parameter is stored in the entry, whether or not * an entry already exists for the specified key. * * As a side effect, this method expunges all stale entries in the * "run" containing the stale entry. (A run is a sequence of entries * between two null slots.) * * @param key the key * @param value the value to be associated with key * @param staleSlot index of the first stale entry encountered while * searching for key. */ private void replaceStaleEntry(ThreadLocal<?> key, Object value, int staleSlot) { Entry[] tab = table; int len = tab.length; Entry e; // Back up to check for prior stale entry in current run. // We clean out whole runs at a time to avoid continual // incremental rehashing due to garbage collector freeing // up refs in bunches (i.e., whenever the collector runs). int slotToExpunge = staleSlot; for (int i = prevIndex(staleSlot, len); (e = tab[i]) != null; i = prevIndex(i, len)) if (e.get() == null) slotToExpunge = i; // Find either the key or trailing null slot of run, whichever // occurs first for (int i = nextIndex(staleSlot, len); (e = tab[i]) != null; i = nextIndex(i, len)) { ThreadLocal<?> k = e.get(); // If we find key, then we need to swap it // with the stale entry to maintain hash table order. // The newly stale slot, or any other stale slot // encountered above it, can then be sent to expungeStaleEntry // to remove or rehash all of the other entries in run. if (k == key) { e.value = value; tab[i] = tab[staleSlot]; tab[staleSlot] = e; // Start expunge at preceding stale entry if it exists if (slotToExpunge == staleSlot) slotToExpunge = i; cleanSomeSlots(expungeStaleEntry(slotToExpunge), len); return; } // If we didn't find stale entry on backward scan, the // first stale entry seen while scanning for key is the // first still present in the run. if (k == null && slotToExpunge == staleSlot) slotToExpunge = i; } // If key not found, put new entry in stale slot tab[staleSlot].value = null; tab[staleSlot] = new Entry(key, value); // If there are any other stale entries in run, expunge them if (slotToExpunge != staleSlot) cleanSomeSlots(expungeStaleEntry(slotToExpunge), len); } /** * Expunge a stale entry by rehashing any possibly colliding entries * lying between staleSlot and the next null slot. This also expunges * any other stale entries encountered before the trailing null. See * Knuth, Section 6.4 * * @param staleSlot index of slot known to have null key * @return the index of the next null slot after staleSlot * (all between staleSlot and this slot will have been checked * for expunging). */ private int expungeStaleEntry(int staleSlot) { Entry[] tab = table; int len = tab.length; // expunge entry at staleSlot tab[staleSlot].value = null; tab[staleSlot] = null; size--; // Rehash until we encounter null Entry e; int i; for (i = nextIndex(staleSlot, len); (e = tab[i]) != null; i = nextIndex(i, len)) { ThreadLocal<?> k = e.get(); if (k == null) { e.value = null; tab[i] = null; size--; } else { int h = k.threadLocalHashCode & (len - 1); if (h != i) { tab[i] = null; // Unlike Knuth 6.4 Algorithm R, we must scan until // null because multiple entries could have been stale. while (tab[h] != null) h = nextIndex(h, len); tab[h] = e; } } } return i; } /** * Heuristically scan some cells looking for stale entries. * This is invoked when either a new element is added, or * another stale one has been expunged. It performs a * logarithmic number of scans, as a balance between no * scanning (fast but retains garbage) and a number of scans * proportional to number of elements, that would find all * garbage but would cause some insertions to take O(n) time. * * @param i a position known NOT to hold a stale entry. The * scan starts at the element after i. * * @param n scan control: {@code log2(n)} cells are scanned, * unless a stale entry is found, in which case * {@code log2(table.length)-1} additional cells are scanned. * When called from insertions, this parameter is the number * of elements, but when from replaceStaleEntry, it is the * table length. (Note: all this could be changed to be either * more or less aggressive by weighting n instead of just * using straight log n. But this version is simple, fast, and * seems to work well.) * * @return true if any stale entries have been removed. */ private boolean cleanSomeSlots(int i, int n) { boolean removed = false; Entry[] tab = table; int len = tab.length; do { i = nextIndex(i, len); Entry e = tab[i]; if (e != null && e.get() == null) { n = len; removed = true; i = expungeStaleEntry(i); } } while ( (n >>>= 1) != 0); return removed; } /** * Re-pack and/or re-size the table. First scan the entire * table removing stale entries. If this doesn't sufficiently * shrink the size of the table, double the table size. */ private void rehash() { expungeStaleEntries(); // Use lower threshold for doubling to avoid hysteresis if (size >= threshold - threshold / 4) resize(); } /** * Double the capacity of the table. */ private void resize() { Entry[] oldTab = table; int oldLen = oldTab.length; int newLen = oldLen * 2; Entry[] newTab = new Entry[newLen]; int count = 0; for (int j = 0; j < oldLen; ++j) { Entry e = oldTab[j]; if (e != null) { ThreadLocal<?> k = e.get(); if (k == null) { e.value = null; // Help the GC } else { int h = k.threadLocalHashCode & (newLen - 1); while (newTab[h] != null) h = nextIndex(h, newLen); newTab[h] = e; count++; } } } setThreshold(newLen); size = count; table = newTab; } /** * Expunge all stale entries in the table. */ private void expungeStaleEntries() { Entry[] tab = table; int len = tab.length; for (int j = 0; j < len; j++) { Entry e = tab[j]; if (e != null && e.get() == null) expungeStaleEntry(j); } } } }
ThreadLocal 暴露了5个基本的操作和构造方法，主要的功能有：构造方法、设值方法、取值方法和资源回收；
1. 构造方法
ThreadLocal 是一个泛型类，只提供了一个构造方法，通过泛型可以指定要存储的值的类型；这个构造方法通常可以单独使用，也可以配合initialValue方法在实例化对象时提供一个初始值。
2.设值方法set
保存数据可以使用set方法，多次调用set方法不会保存多个数据，而是会覆盖掉。一个ThreadLocal只能保存一个数据：
public void set(T value) { Thread t = Thread.currentThread(); // 获取当前线程 ThreadLocalMap map = getMap(t); // 拿到当前线程中的ThreadLocalMap if (map != null) map.set(this, value); // 线程中存在ThreadLocalMap，设值 else createMap(t, value); // 线程中不存在ThreadLocalMap，创建后再设值 }
3.取值方法get
在没有使用set方法设值之前，调用get方法获取到的值将是initialValue方法设置的值（如果此方法未被覆盖则返回null）。调用set方法设值之后，返回的就是设置的值。
public T get() { Thread t = Thread.currentThread(); // 获取当前线程 ThreadLocalMap map = getMap(t); // 拿到当前线程保存的ThreadLocalMap if (map != null) { ThreadLocalMap.Entry e = map.getEntry(this); // 这里传入的this就是当前的ThreadLocal对象，拿到ThreadLocal对应的值 if (e != null) { @SuppressWarnings("unchecked") T result = (T)e.value; // 拿到ThreadLocal的值 return result; } } return setInitialValue(); // 调用setInitialValue方法返回初始值 }
4.资源回收remove
当我们不再需要保存的数据时，应该通过remove方法将当前线程中保存的值移除掉使对象得到GC（调用remove方法将把ThreadLocal对象从当前线程的ThreadLocalMap移除）：
public void remove() { ThreadLocalMap m = getMap(Thread.currentThread()); // 拿到当前线程中的ThreadLocalMap if (m != null) { m.remove(this); // 从ThreadLocalMap移除key为当前ThreadLocal对象的记录 } }
调用remove方法会清空使用set方法设置的值，此时如果再次调用了get方法，由于ThreadLocal对应的记录已经不存在，所以将会执行return setInitialValue();这段代码，这里将会调用initialValue方法从而返回初始值。
四、ThreadLocal 内存泄漏问题
ThreadLocalMap中的Entry中，ThreadLocal作为key，是作为弱引用进行存储的。当ThreadLocal不再被作为强引用持有时，会被GC回收，这时ThreadLocalMap对应的ThreadLocal就变成了null。而根据文档所叙述的，当key == null时，这时就可以默认该键不再被引用，该Entry就可以被直接清除，该清除行为会在Entry本身的set()/get()/remove()中被调用。
一般情况下，线程在执行结束时，自然也会消除其对value的引用，使得Value能够被GC回收。但是在使用线程池或者其他特殊情况下的时候，会存在线程复用的情况，这时候Value的值依然能获取到，可能就存在内存泄漏的隐患了。所以我们推荐通过手动将value的值设置为null（即调用ThreadLocal.remove()方法）以规避内存泄漏的风险。
五、推荐资料
2. 《Java并发编程的艺术》