java.lang.Objectjava.lang.ThreadLocal<T>
public class ThreadLocal<T>
This class provides thread-local variables. These variables differ from their normal counterparts in that each thread that accesses one (via its get or set method) has its own, independently initialized copy of the variable. 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).
For example, in the class below, the private static ThreadLocal instance ( serialNum ) maintains a "serial number" for each thread that invokes the class's static SerialNum.get() method, which returns the current thread's serial number. (A thread's serial number is assigned the first time it invokes SerialNum.get() , and remains unchanged on subsequent calls.)
public class SerialNum { // The next serial number to be assigned private static int nextSerialNum = 0; private static ThreadLocal serialNum = new ThreadLocal() { protected synchronized Object initialValue() { return new Integer(nextSerialNum++); } }; public static int get() { return ((Integer) (serialNum.get())).intValue(); } }
Each thread holds an implicit reference to its copy of a thread-local variable as long as the thread is alive and the 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).
Constructor Summary | |
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ThreadLocal
() Creates a thread local variable. |
Method Summary | |
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T |
get
() Returns the value in the current thread's copy of this thread-local variable. |
protected T |
initialValue
() Returns the current thread's ![]() ![]() |
void |
remove
() Removes the ![]() ![]() ![]() |
void |
set
(
T
value) Sets the current thread's copy of this thread-local variable to the specified value. |
Methods inherited from class java.lang. Object |
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clone , equals , finalize , getClass , hashCode , notify , notifyAll , toString , wait , wait , wait |
Constructor Detail |
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public ThreadLocal()
Method Detail |
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protected T initialValue()
This implementation simply returns
null
; if the programmer desires thread-local variables to
have an initial
be initialized to some
value other than
null
,
ThreadLocal
must be subclassed, and this method overridden. Typically, an anonymous inner class will be used.
Typical implementations of
initialValue
will invoke an appropriate constructor and return the newly constructed object.
public T get()
public void set(T value)
public void remove()