关于ThreadLocal分析和总结

什么是ThreadLocal？

       首先，ThreadLocal 不是用来解决共享对象的多线程访问问题的，一般情况下，通过ThreadLocal.set() 到线程中的对象是该线程自己使用的对象，其他线程是不需要访问的，也访问不到的。各个线程中访问的是不同的对象。        另外，说ThreadLocal使得各线程能够保持各自独立的一个对象，并不是通过ThreadLocal.set()来实现的，而是通过每个线程中的new 对象 的操作来创建的对象，每个线程创建一个，不是什么对象的拷贝或副本。通过ThreadLocal.set()将这个新创建的对象的引用保存到各线程的自己的一个map中，每个线程都有这样一个map，执行ThreadLocal.get()时，各线程从自己的map中取出放进去的对象，因此取出来的是各自自己线程中的对象，ThreadLocal实例是作为map的key来使用的。

        总之，ThreadLocal不是用来解决对象共享访问问题的，而主要是提供了保持对象的方法和避免参数传递的方便的对象访问方式。归纳了两点：            当然如果要把本来线程共享的对象通过ThreadLocal.set()放到线程中也可以，可以实现避免参数传递的访问方式，但是要注意get()到的是那同一个共享对象，并发访问问题要靠其他手段来解决。但一般来说线程共享的对象通过设置为某类的静态变量就可以实现方便的访问了，似乎没必要放到线程中。          ThreadLocal的应用场合，我觉得最适合的是按线程多实例（每个线程对应一个实例）的对象的访问，并且这个对象很多地方都要用到。

public class ThreadLocal<T> {
/**
* ThreadLocals rely on per-thread 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. Accessed only by like-named method.
*/
private static int nextHashCode = 0;
/**
* 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;
/**
* Compute the next hash code. The static synchronization used here
* should not be a performance bottleneck. When ThreadLocals are
* generated in different threads at a fast enough rate to regularly
* contend on this lock, memory contention is by far a more serious
* problem than lock contention.
*/
private static synchronized int nextHashCode() {
int h = nextHashCode;
nextHashCode = h + HASH_INCREMENT;
return h;
}
/**
* Creates a thread local variable.
*/
public ThreadLocal() {
}
/**
* Returns the value in the current thread's copy of this thread-local
* variable.
Creates and initializes the copy if this is the first time
* the thread has called this 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)
return (T)map.get(this);
// Maps are constructed lazily.
if the map for this thread
// doesn't exist, create it, with this ThreadLocal and its
// initial value as its only entry.
T value = initialValue();
createMap(t, value);
return value;
}
/**
* Sets the current thread's copy of this thread-local variable
* to the specified value.
Many applications will have no need for
* this functionality, relying solely on the {@link #initialValue}
* method to set the values of thread-locals.
*
* @param value the value to be stored in the current threads' 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
* @param map the map to store.
*/
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
.......
/**
* 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 {
........
}
}

   针对上面上面的代码在1.6中采用原子性操作(AtomicInteger )，更加安全。

JDK 1.6代码如下：

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 <tt>initialValue</tt> 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 <tt>null</tt>; if the
* programmer desires thread-local variables to have an initial
* value other than <tt>null</tt>, <tt>ThreadLocal</tt> 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.
*/
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)
return (T)e.value;
}
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
* <tt>initialValue</tt> 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
* @param map the map to store.
*/
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();
}
/**
* 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 {
...
}
}

可以看到ThreadLocal类中的变量只有这3个int型：

/**
* 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;

private final int threadLocalHashCode = nextHashCode();

那么nextHashCode()做了什么呢：

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

/**
* 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;
}

Synchronized还是ThreadLocal? ThreadLocal以空间换取时间，提供了一种非常简便的多线程实现方式。因为多个线程并发访问无需进行等待，所以使用ThreadLocal会获得更大的性能。虽然使用ThreadLocal会带来更多的内存开销，但这点开销是微不足道的。因为保存在ThreadLocal中的对象，通常都是比较小的对象。另外使用ThreadLocal不能使用原子类型，只能使用Object类型。ThreadLocal的使用比synchronized要简单得多。     ThreadLocal和Synchonized都用于解决多线程并发访问。但是ThreadLocal与synchronized有本质的区别。synchronized是利用锁的机制，使变量或代码块在某一时该只能被一个线程访问。而ThreadLocal为每一个线程都提供了变量的副本，使得每个线程在某一时间访问到的并不是同一个对象，这样就隔离了多个线程对数据的数据共享。而Synchronized却正好相反，它用于在多个线程间通信时能够获得数据共享。 Synchronized用于线程间的数据共享，而ThreadLocal则用于线程间的数据隔离。 当然ThreadLocal并不能替代synchronized,它们处理不同的问题域。Synchronized用于实现同步机制，比ThreadLocal更加复杂。

ThreadLocal和线程同步机制相比有什么优势呢？ThreadLocal和线程同步机制都是为了解决多线程中相同变量的访问冲突问题。

　　在同步机制中，通过对象的锁机制保证同一时间只有一个线程访问变量。这时该变量是多个线程共享的，使用同步机制要求程序慎密地分析什么时候对变量进行读写，什么时候需要锁定某个对象，什么时候释放对象锁等繁杂的问题，程序设计和编写难度相对较大。

　　而ThreadLocal则从另一个角度来解决多线程的并发访问。ThreadLocal会为每一个线程提供一个独立的变量副本，从而隔离了多个线程对数据的访问冲突。因为每一个线程都拥有自己的变量副本，从而也就没有必要对该变量进行同步了。ThreadLocal提供了线程安全的共享对象，在编写多线程代码时，可以把不安全的变量封装进ThreadLocal。

　　由于ThreadLocal中可以持有任何类型的对象，低版本JDK所提供的get()返回的是Object对象，需要强制类型转换。但JDK 5.0通过泛型很好的解决了这个问题，在一定程度地简化ThreadLocal的使用。

　　概括起来说，对于多线程资源共享的问题，同步机制采用了“以时间换空间”的方式，而ThreadLocal采用了“以空间换时间”的方式。前者仅提供一份变量，让不同的线程排队访问，而后者为每一个线程都提供了一份变量，因此可以同时访问而互不影响。

　　Spring使用ThreadLocal解决线程安全问题

　　我们知道在一般情况下，只有无状态的Bean才可以在多线程环境下共享，在Spring中，绝大部分Bean都可以声明为singleton作用域。就是因为Spring对一些Bean（如RequestContextHolder、        TransactionSynchronizationManager、LocaleContextHolder等）中非线程安全状态采用ThreadLocal进行处理，让它们也成为线程安全的状态，因为有状态的Bean就可以在多线程中共享了。

　　一般的Web应用划分为展现层、服务层和持久层三个层次，在不同的层中编写对应的逻辑，下层通过接口向上层开放功能调用。在一般情况下，从接收请求到返回响应所经过的所有程序调用都同属于一个线程，如图9‑2所示：

图1同一线程贯通三层

　　这样你就可以根据需要，将一些非线程安全的变量以ThreadLocal存放，在同一次请求响应的调用线程中，所有关联的对象引用到的都是同一个变量。

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