JavaTM 2 Platform Std. Ed. v1.4.2
java.lang
Class ThreadLocal
java.lang.Object
java.lang.ThreadLocal
- Direct Known Subclasses:
- InheritableThreadLocal
- public class ThreadLocal
- extends Object
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).
- Since:
- 1.2
Method Summary |
Object |
get()
Returns the value in the current thread's copy of this thread-local
variable. |
protected Object |
initialValue()
Returns the current thread's initial value for this thread-local
variable. |
void |
set(Object value)
Sets the current thread's copy of this thread-local variable
to the specified value. |
Methods inherited from class java.lang.Object |
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
ThreadLocal
public ThreadLocal()
initialValue
protected Object initialValue()
- Returns the current thread's initial value for this thread-local
variable. This method will be invoked at most once per accessing
thread for each thread-local, the first time the thread accesses the
variable with the
get() method. The initialValue
method will not be invoked in a thread if the thread invokes the set(Object) method prior to the get method.
This implementation simply returns null; if the programmer
desires thread-local variables to 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.
- Returns:
- the initial value for this thread-local
get
public Object get()
- 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.
- Returns:
- the current thread's value of this thread-local
set
public void set(Object 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
initialValue()
method to set the values of thread-locals.
- Parameters:
value - the value to be stored in the current threads' copy of
this thread-local.
Copyright 2003 Sun Microsystems, Inc. All rights reserved
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