java.lang: public class: ThreadLocal

java.lang
public class: ThreadLocal [javadoc | source]

java.lang.Object
   java.lang.ThreadLocal

Direct Known Subclasses:
ThreadLocalHoldCounter, InheritableThreadLocal

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, the class below generates unique identifiers local to each thread. A thread's id is assigned the first time it invokes ThreadId.get() and remains unchanged on subsequent calls.

import java.util.concurrent.atomic.AtomicInteger;

public class ThreadId {
// Atomic integer containing the next thread ID to be assigned
private static final AtomicInteger nextId = new AtomicInteger(0);

// Thread local variable containing each thread's ID
private static final ThreadLocal<Integer> threadId =
new ThreadLocal<Integer>() {
@Override protected Integer initialValue() {
return nextId.getAndIncrement();
}
};

// Returns the current thread's unique ID, assigning it if necessary
public static int get() {
return threadId.get();
}
}

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

since: 1.2 -

Nested Class Summary:
static class	ThreadLocal.ThreadLocalMap	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.

Constructor:
public ThreadLocal() { } Creates a thread local variable.

Constructor:

 public ThreadLocal() {

}

Creates a thread local variable.

Method from java.lang.ThreadLocal Summary:
childValue, createInheritedMap, createMap, get, getMap, initialValue, remove, set

Methods from java.lang.Object:
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method from java.lang.ThreadLocal Detail:
T childValue(T parentValue) { throw new UnsupportedOperationException(); } 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.
static ThreadLocal.ThreadLocalMap createInheritedMap(ThreadLocal.ThreadLocalMap parentMap) { return new ThreadLocalMap(parentMap); } Factory method to create map of inherited thread locals. Designed to be called only from Thread constructor.
void createMap(Thread t, T firstValue) { t.threadLocals = new ThreadLocalMap(this, firstValue); } Create the map associated with a ThreadLocal. Overridden in InheritableThreadLocal.
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(); } 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 #initialValue method.
ThreadLocal.ThreadLocalMap getMap(Thread t) { return t.threadLocals; } Get the map associated with a ThreadLocal. Overridden in InheritableThreadLocal.
protected T initialValue() { return null; } 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 #get method, unless the thread previously invoked the #set method, in which case the `initialValue` 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 #remove followed by #get . This implementation simply returns `null`; if the programmer desires thread-local variables to have an initial value other than `null`, `ThreadLocal` must be subclassed, and this method overridden. Typically, an anonymous inner class will be used.
public void remove() { ThreadLocalMap m = getMap(Thread.currentThread()); if (m != null) m.remove(this); } 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 #initialValue method, unless its value is {@linkplain #set set} by the current thread in the interim. This may result in multiple invocations of the `initialValue` method in the current thread.
public void set(T value) { Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) map.set(this, value); else createMap(t, 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 #initialValue method to set the values of thread-locals.

Method from java.lang.ThreadLocal Detail:

 T childValue(T parentValue) {
        throw new UnsupportedOperationException();
}

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.

 static ThreadLocal.ThreadLocalMap createInheritedMap(ThreadLocal.ThreadLocalMap parentMap) {
        return new ThreadLocalMap(parentMap);
}

Factory method to create map of inherited thread locals. Designed to be called only from Thread constructor.

  void createMap(Thread t,
    T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
}

Create the map associated with a ThreadLocal. Overridden in InheritableThreadLocal.

 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();
}

#initialValue

 ThreadLocal.ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
}

Get the map associated with a ThreadLocal. Overridden in InheritableThreadLocal.

 protected T initialValue() {
        return null;
}

initialValue

This implementation simply returns null; if the programmer desires thread-local variables to have an initial value other than null, ThreadLocal must be subclassed, and this method overridden. Typically, an anonymous inner class will be used.

 public  void remove() {
         ThreadLocalMap m = getMap(Thread.currentThread());
         if (m != null)
             m.remove(this);
}

#initialValue

initialValue

 public  void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
}

#initialValue