ThreadLocal (Java Platform SE 8 ) (original) (raw)

• 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 itsget 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 threadId =
new ThreadLocal() {
@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).
Since:
1.2

• • Constructor Summary

  Constructors

  Constructor	Description
  ThreadLocal()	Creates a thread local variable.

  • Method Summary

  All Methods Static Methods Instance Methods Concrete Methods

  Modifier and Type	Method	Description
  T	get()	Returns the value in the current thread's copy of this thread-local variable.
  protected T	initialValue()	Returns the current thread's "initial value" for this thread-local variable.
  void	remove()	Removes the current thread's value for this thread-local variable.
  void	set(T value)	Sets the current thread's copy of this thread-local variable to the specified value.
  static ThreadLocal	withInitial(Supplier<? extends S> supplier)	Creates a thread local variable.

     * ### Methods inherited from class java.lang.[Object](../../java/lang/Object.html "class in java.lang")  
     `[clone](../../java/lang/Object.html#clone--), [equals](../../java/lang/Object.html#equals-java.lang.Object-), [finalize](../../java/lang/Object.html#finalize--), [getClass](../../java/lang/Object.html#getClass--), [hashCode](../../java/lang/Object.html#hashCode--), [notify](../../java/lang/Object.html#notify--), [notifyAll](../../java/lang/Object.html#notifyAll--), [toString](../../java/lang/Object.html#toString--), [wait](../../java/lang/Object.html#wait--), [wait](../../java/lang/Object.html#wait-long-), [wait](../../java/lang/Object.html#wait-long-int-)`

• • Constructor Detail

     * #### ThreadLocal  
     public ThreadLocal()  
     Creates a thread local variable.  
     See Also:  
     [withInitial(java.util.function.Supplier)](../../java/lang/ThreadLocal.html#withInitial-java.util.function.Supplier-)  

  • Method Detail

    * #### initialValue  
    protected [T](../../java/lang/ThreadLocal.html "type parameter in ThreadLocal") 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 [get()](../../java/lang/ThreadLocal.html#get--) method, unless the thread previously invoked the [set(T)](../../java/lang/ThreadLocal.html#set-T-) 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()](../../java/lang/ThreadLocal.html#remove--) followed by [get()](../../java/lang/ThreadLocal.html#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.  
    Returns:  
    the initial value for this thread-local  
    * #### withInitial  
    public static <S> [ThreadLocal](../../java/lang/ThreadLocal.html "class in java.lang")<S> withInitial([Supplier](../../java/util/function/Supplier.html "interface in java.util.function")<? extends S> supplier)  
    Creates a thread local variable. The initial value of the variable is determined by invoking the `get` method on the `Supplier`.  
    Type Parameters:  
    `S` \- the type of the thread local's value  
    Parameters:  
    `supplier` \- the supplier to be used to determine the initial value  
    Returns:  
    a new thread local variable  
    Throws:  
    `[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang")` \- if the specified supplier is null  
    Since:  
    1.8  
    * #### get  
    public [T](../../java/lang/ThreadLocal.html "type parameter in ThreadLocal") 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 [initialValue()](../../java/lang/ThreadLocal.html#initialValue--) method.  
    Returns:  
    the current thread's value of this thread-local  
    * #### set  
    public void set([T](../../java/lang/ThreadLocal.html "type parameter in ThreadLocal") 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()](../../java/lang/ThreadLocal.html#initialValue--) method to set the values of thread-locals.  
    Parameters:  
    `value` \- the value to be stored in the current thread's copy of this thread-local.  
    * #### remove  
    public void remove()  
    Removes the current thread's value for this thread-local variable. If this thread-local variable is subsequently[read](../../java/lang/ThreadLocal.html#get--) by the current thread, its value will be reinitialized by invoking its [initialValue()](../../java/lang/ThreadLocal.html#initialValue--) method, unless its value is [set](../../java/lang/ThreadLocal.html#set-T-) by the current thread in the interim. This may result in multiple invocations of the`initialValue` method in the current thread.  
    Since:  
    1.5

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