ThreadLocal源码分析
2024-10-15 21:56:55
1、概述
ThreadLocal，可以理解为线程的局部变量，作用就是为每一个使用该变量的线程都提供一个变量值的副本，每一个线程都可以独立地改变自己的副本，而不会和其它线程的副本冲突。
ThreadLocal是如何做到为每一个线程维护变量的副本的呢？
每个线程中都有一个ThreadLocalMap（Thread.threadLocals），用于存储每一个线程的变量的副本。
ThreadLocalMap使用数组Entry[] table保存ThreadLocal-->Object键值对象，数组保存位置：int i = key.nextHashCode() & (table.length - 1);。
ThreadLocal和Synchonized区别：
都用于解决多线程并发访问。
Synchronized用于线程间的数据共享（使变量或代码块在某一时该只能被一个线程访问），是一种以延长访问时间来换取线程安全性的策略；
而ThreadLocal则用于线程间的数据隔离（为每一个线程都提供了变量的副本），是一种以空间来换取线程安全性的策略。
2、ThreadLocalMap
用于存储每一个线程的变量的副本
/**
* 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 table, resized as necessary.
* table.length MUST always be a power of two.
*/
private Entry[] table; /**
* 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);
} /**
* 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;
}
}
} ......
}
3、ThreadLocal
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
*/
//返回此线程局部变量的当前线程的初始值
protected T initialValue() {
return null;
}
/**
* 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
*/
//返回此线程局部变量的当前线程副本中的值
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = t.threadLocals;
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}
/**
* 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.
*/
//将此线程局部变量的当前线程副本中的值设置为指定值
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = t.threadLocals;
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
*/
//移除此线程局部变量的值
public void remove() {
ThreadLocalMap m = Thread.currentThread().threadLocals;
if (m != null)
m.remove(this);
}
/**
* 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);
}
......
}
为什么取HASH_INCREMENT = 0x61c88647?
（可以阅读Why 0x61c88647?）
由来：
This number represents the golden ratio (sqrt(5)-1) times two to the power of 31. The result is then a golden number, either 2654435769 or -1640531527. You can see the calculation here:
long l1 = (long) ((1L << 31) * (Math.sqrt(5) - 1)); //Math.sqrt(5) - 1 = 1.2360679774997898
System.out.println("as 32 bit unsigned: " + l1); // int i1 = (int) l1;
System.out.println("as 32 bit signed: " + i1); //-1640531527 = -0x61c88647
与fibonacci hashing(斐波那契散列法)以及黄金分割有关，特殊的哈希码0x61c88647大大降低碰撞的几率，能让哈希码能均匀的分布在2的N次方的数组里。
key.threadLocalHashCode & (len-1)，ThreadLocalMap 中 Entry[] table 的大小必须是2的N次方呀(len = 2^N)，那 len-1 的二进制表示就是低位连续的N个1， 那 key.threadLocalHashCode & (len-1) 的值就是 threadLocalHashCode 的低N位。
测试：
private static AtomicInteger nextHashCode = new AtomicInteger();
private static final int HASH_INCREMENT = 0x61c88647;
private static int nextHashCode() {
return nextHashCode.getAndAdd(HASH_INCREMENT);
}
public static void main(String[] args) {
for (int j = 0; j < 5; j++) {
int size = 2 << j;
// hash = 0;
int[] indexArray = new int[size];
for (int i = 0; i < size; i++) {
indexArray[i] = nextHashCode() & (size - 1);
}
System.out.println("indexs = "+ Arrays.toString(indexArray));
}
}
结果：
indexs = [0, 1]
indexs = [2, 1, 0, 3]
indexs = [2, 1, 0, 7, 6, 5, 4, 3]
indexs = [2, 9, 0, 7, 14, 5, 12, 3, 10, 1, 8, 15, 6, 13, 4, 11]
indexs = [18, 25, 0, 7, 14, 21, 28, 3, 10, 17, 24, 31, 6, 13, 20, 27, 2, 9, 16, 23, 30, 5, 12, 19, 26, 1, 8, 15, 22, 29, 4, 11]
没有看到重复的索引值，要哈希表的大小是2的N次方,那么基本上可以保证每次计算出的index值都不会重复。
为什么HashCode不直接用自增的方式(HASH_INCREMENT=1)?
我的理解是，随着不用的 ThreadLocal 变量被回收掉，这种自增的方式的性能会越来越差，因为临近的 slot 为空的可能性很小。而 ThreadLocal 实际所采用的方式，其下标是在跳跃分布，这样即使出现冲突，在临近找到空 slot 的可能性更大一些，性能也会更好。
4、例子
Android Looper的实现：
public final class Looper {
......
// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static Looper myLooper() {
return sThreadLocal.get();
}
......
}
ThreadId:
维护每个线程的id
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();
}
}
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