ThreadLocal
2010-10-06 22:32 wansishuang 阅读(400) 评论(0) 编辑 收藏 举报
Class: java.lang.ThreadLocal<T>
- public class ThreadLocal<T>
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).
Methods
-
ThreadLocaltop
public ThreadLocal()Creates a thread local variable. -
gettop
public T get()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 ThreadLocal.initialValue() method. -
initialValuetop
protected T initialValue()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 ThreadLocal.get() method, unless the thread previously invoked the ThreadLocal.set(java.lang.Object) 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 ThreadLocal.remove() followed by ThreadLocal.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.
-
removetop
public void remove()Removes the current thread's value for this thread-local variable. If this thread-local variable is subsequently read by the current thread, its value will be reinitialized by invoking its ThreadLocal.initialValue() method, unless its value is set by the current thread in the interim. This may result in multiple invocations of the initialValue method in the current thread. -
settop
public void set(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 ThreadLocal.initialValue() method to set the values of thread-locals.
