ThreadLocal 源码剖析
ThreadLocal是Java语言提供的用于支持线程局部变量的类。所谓的线程局部变量,就是仅仅只能被本线程访问,不能在线程之间进行共享访问的变量(每个线程一个拷贝)。在各个Java web的各种框架中ThreadLocal几乎已经被用烂了,spring中有使用,mybatis中也有使用,hibernate中也有使用,甚至我们写个分页也用ThreadLocal来传递参数......这也从侧面说明了ThreadLocal十分的给力。
从使用者的角度而言,一般我们可以将ThreadLocal看做是一个:ConcurrentHashMap<Thread, Object>,以Thread引用为key, 来保存本线程的局部变量。但是从实现的角度而言,ThreadLocal的实现根本就不是这样的。下面从源码分析ThreadLocal的实现。
1. 既然是线程局部变量,那么理所当然就应该存储在自己的线程对象中,我们可以从 Thread 的源码中找到线程局部变量存储的地方:
public class Thread implements Runnable {
/* Make sure registerNatives is the first thing <clinit> does. */
private static native void registerNatives();
static {
registerNatives();
}
// ... ...
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
/*
* InheritableThreadLocal values pertaining to this thread. This map is
* maintained by the InheritableThreadLocal class.
*/
ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
我们可以看到线程局部变量是存储在Thread对象的 threadLocals 属性中,而 threadLocals 属性是一个 ThreadLocal.ThreadLocalMap 对象。
2. 我们接着看 ThreadLocal.ThreadLocalMap 是何方神圣
/**
* 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;
// ... ...
/**
* 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);
}
可以看到ThreadLocal.ThreadLocalMap 是 ThreadLocal 的一个静态内部类。ThreadLocalMap从字面上就可以看出这是一个保存ThreadLocal对象的map(其实是以它为Key),没错,不过是经过了两层包装的ThreadLocal对象。第一层包装是使用 WeakReference<ThreadLocal<?>> 将ThreadLocal对象变成一个弱引用的对象;第二层包装是 定义了一个专门的类 Entry 来扩展 WeakReference<ThreadLocal<?>>:
static class Entry extends WeakReference<ThreadLocal<?>> {
/** The value associated with this ThreadLocal. */
Object value;
Entry(ThreadLocal<?> k, Object v) {
super(k);
value = v;
}
}
类 Entry 很显然是一个保存map键值对的实体,ThreadLocal<?>为key, 要保存的线程局部变量的值为value。super(k)调用的WeakReference的构造函数,表示将ThreadLocal<?>对象转换成弱引用对象,用做key。
从 ThreadLocalMap 的构造函数:
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);
}
可以看出,ThreadLocalMap这个map的实现是使用一个数组 private Entry[] table 来保存键值对的实体,初始大小为16,ThreadLocalMap自己实现了如何从 key 到 value 的映射: firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1)
/**
* 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);
}
使用一个 static 的原子属性 AtomicInteger nextHashCode,通过每次增加 HASH_INCREMENT = 0x61c88647 ,然后 & (INITIAL_CAPACITY - 1) 取得在数组 private Entry[] table 中的索引。
3. 我们先看一下 Thread 对象中的 ThreadLocal.ThreadLocalMap threadLocals = null; 如何初始化:
/**
* 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);
}
/**
* 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);
}
ThreadLocal在调用set方法时,如果 getMap(注意是以Thread引用为key) 返回的 t.threadLocals 为null,那么表示该线程的 ThreadLocalMap 还没有初始化,所以调用createMap进行初始化:t.threadLocals = new ThreadLocalMap(this, firstValue);
注意这里使用到了延迟初始化的技术:
/**
* 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);
}
这里仅仅是初始化了16个元素的引用数组,并没有初始化16个 Entry 对象。而是一个线程中有多少个线程局部对象要保存,那么就初始化多少个 Entry 对象来保存它们。
到了这里,我们可以思考一下,为什么要这样实现了。为什么要用 ThreadLocalMap 来保存线程局部对象呢?原因是一个线程拥有的的局部对象可能有很多,这样实现的话,那么不管你一个线程拥有多少个局部变量,都是使用同一个 ThreadLocalMap 来保存的,ThreadLocalMap 中 private Entry[] table 的初始大小是16。超过容量的2/3时,会扩容。
4. 我们在看一下 ThreadLocal.set 方法:
/**
* 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);
}
我们看到是以当前 thread 的引用为 key, 获得 ThreadLocalMap ,然后调用 map.set(this, value); 保存进 private Entry[] table :
/**
* 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();
}
5. ThreadLocal 涉及到的两个层面的内存自动回收
1)在 ThreadLocal 层面的内存回收:
/*
* 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).
当线程死亡时,那么所有的保存在的线程局部变量就会被回收,其实这里是指线程Thread对象中的 ThreadLocal.ThreadLocalMap threadLocals 会被回收,这是显然的。
2)ThreadLocalMap 层面的内存回收:
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/** * 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. */ |
如果线程可以活很长的时间,并且该线程保存的线程局部变量有很多(也就是 Entry 对象很多),那么就涉及到在线程的生命期内如何回收 ThreadLocalMap 的内存了,不然的话,Entry对象越多,那么ThreadLocalMap 就会越来越大,占用的内存就会越来越多,所以对于已经不需要了的线程局部变量,就应该清理掉其对应的Entry对象。使用的方式是,Entry对象的key是WeakReference 的包装,当ThreadLocalMap 的 private Entry[] table,已经被占用达到了三分之二时 threshold = 2/3(也就是线程拥有的局部变量超过了10个) ,就会尝试回收 Entry 对象,我们可以看到 ThreadLocalMap.set方法中有下面的代码:
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if (!cleanSomeSlots(i, sz) && sz >= threshold) rehash(); |
cleanSomeSlots 就是进行回收内存:
/**
* 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;
}
e.get() == null 调用的是 Entry 的父类 WeakReference<ThreadLocal<?>> 的方法:
/**
* Returns this reference object's referent. If this reference object has
* been cleared, either by the program or by the garbage collector, then
* this method returns <code>null</code>.
*
* @return The object to which this reference refers, or
* <code>null</code> if this reference object has been cleared
*/
public T get() {
return this.referent;
}
返回 null ,表示 Entry 的 key 已经被回收了,所以可以回收该 Entry 对象了:expungeStaleEntry(i)
/**
* 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--;
6. ThreadLocal常用的接口:
1)需要制定初始值时,可以覆盖protected T initialValue()方法;
2)public T get();
3)public void set(T value);
4)public void remove();
7. 总结
1)一个线程中的所有的局部变量其实存储在该线程自己的同一个map属性中;
2)线程死亡时,线程局部变量会自动回收内存;
3)线程局部变量时通过一个 Entry 保存在map中,该Entry 的key是一个 WeakReference包装的ThreadLocal, value为线程局部变量;
key 到 value 的映射是通过:ThreadLocal.threadLocalHashCode & (INITIAL_CAPACITY - 1) 来完成的;
4)当线程拥有的局部变量超过了容量的2/3(没有扩大容量时是10个),会涉及到ThreadLocalMap中Entry的回收;
那么实现机制是如何的呢?
1、每个Thread对象内部都维护了一个ThreadLocalMap这样一个ThreadLocal的Map,可以存放若干个ThreadLocal。
2、当我们在调用get()方法的时候,先获取当前线程,然后获取到当前线程的ThreadLocalMap对象,如果非空,那么取出ThreadLocal的value,否则进行初始化,初始化就是将initialValue的值set到ThreadLocal中。
3、当我们调用set()方法的时候,很常规,就是将值设置进ThreadLocal中。
4、总结:当我们调用get方法的时候,其实每个当前线程中都有一个ThreadLocal。每次获取或者设置都是对该ThreadLocal进行的操作,是与其他线程分开的。
5、应用场景: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。
6、其实说再多也不如看一下源码来得清晰。如果要看源码,其中涉及到一个WeakReference和一个Map,这两个地方需要了解下,这两个东西分别是a.Java的弱引用,也就是GC的时候会销毁该引用所包裹(引用)的对象,这个threadLocal作为key可能被销毁,但是只要我们定义成他的类不卸载,tl这个强引用就始终引用着这个ThreadLocal的,永远不会被gc掉。b.和HashMap差不多。
事实上,从本质来讲,就是每个线程都维护了一个map,而这个map的key就是threadLocal,而值就是我们set的那个值,每次线程在get的时候,都从自己的变量中取值,既然从自己的变量中取值,那肯定就不存在线程安全问题,总体来讲,ThreadLocal这个变量的状态根本没有发生变化,他仅仅是充当一个key的角色,另外提供给每一个线程一个初始值。如果允许的话,我们自己就能实现一个这样的功能,只不过恰好JDK就已经帮我们做了这个事情。
1. ThreadLocal只是操作Thread中的ThreadLocalMap对象的集合.
2. ThreadLocalMap变量属于线程的内部属性,不同的线程拥有完全不同的ThreadLocalMap变量.
3. 线程中的ThreadLocalMap变量的值是在ThreadLocal对象进行set或者get操作时创建的.
4. 使用当前线程的ThreadLocalMap的关键在于使用当前的ThreadLocal的实例作为key来存储value值.
5. ThreadLocal模式至少从两个方面完成了数据访问隔离,即纵向隔离(线程与线程之间的ThreadLocalMap不同)和横向隔离(不同的ThreadLocal实例之间的互相隔离).