java.lang.ThreadLocal Maven / Gradle / Ivy
/*
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This work corresponds to the API signatures of JSR 219: Foundation
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JSR 219 specification, which is available at
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*/
package java.lang;
import java.lang.ref.*;
/**
* 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).
*
* @author Josh Bloch and Doug Lea
* @version 1.21, 03/12/05
* @since 1.2
*/
public class ThreadLocal
{
public ThreadLocal() { }
/**
* 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 {@link #get()} method. The initialValue
* method will not be invoked in a thread if the thread invokes the {@link
* #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.
*
* @return the initial value for this thread-local
*/
protected java.lang.Object initialValue() {
return null;
}
/**
* 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 java.lang.Object get() {
return null;
}
/**
* 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(java.lang.Object value) { }
}