- Direct Known Subclasses:
InheritableThreadLocal
public class ThreadLocal<T> extends Object
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).
- Since:
- 1.2
-
Constructor Summary
-
Method Summary
Modifier and TypeMethodDescriptionget()
Returns the value in the current thread's copy of this thread-local variable.protected T
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
Sets the current thread's copy of this thread-local variable to the specified value.static <S> ThreadLocal<S>
withInitial(Supplier<? extends S> supplier)
Creates a thread local variable.
-
Constructor Details
-
ThreadLocal
public ThreadLocal()Creates a thread local variable.- See Also:
withInitial(java.util.function.Supplier)
-
-
Method Details
-
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 theget()
method, unless the thread previously invoked theset(T)
method, in which case theinitialValue
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 ofremove()
followed byget()
.This implementation simply returns
null
; if the programmer desires thread-local variables to have an initial value other thannull
,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
Creates a thread local variable. The initial value of the variable is determined by invoking theget
method on theSupplier
.- 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
- if the specified supplier is null- Since:
- 1.8
-
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 theinitialValue()
method.- Returns:
- the current thread's value of this thread-local
-
set
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 theinitialValue()
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 by the current thread, its value will be reinitialized by invoking itsinitialValue()
method, unless its value is set by the current thread in the interim. This may result in multiple invocations of theinitialValue
method in the current thread.- Since:
- 1.5
-