public class ThreadLocal<T> extends Object
get()
and set()
methods)
only affects the state of the object as seen by the currently
executing Thread.
The first time a ThreadLocal object is accessed on a particular
Thread, the state for that Thread's copy of the local variable is set by
executing the method initialValue()
.
An example how you can use this:
class Connection { private static ThreadLocal owner = new ThreadLocal() { public Object initialValue() { return("nobody"); } }; ... }
Now all instances of connection can see who the owner of the currently
executing Thread is by calling owner.get()
. By default any
Thread would be associated with 'nobody'. But the Connection object could
offer a method that changes the owner associated with the Thread on
which the method was called by calling owner.put("somebody")
.
(Such an owner changing method should then be guarded by security checks.)
When a Thread is garbage collected all references to values of the ThreadLocal objects associated with that Thread are removed.
Constructor and Description |
---|
ThreadLocal()
Creates a ThreadLocal object without associating any value to it yet.
|
Modifier and Type | Method and Description |
---|---|
protected void |
finalize()
Called on an object by the Virtual Machine at most once,
at some point after the Object is determined unreachable
but before it is destroyed.
|
T |
get()
Gets the value associated with the ThreadLocal object for the currently
executing Thread.
|
protected T |
initialValue()
Called once per thread on the first invocation of get(), if set() was
not already called.
|
void |
remove()
Removes the value associated with the ThreadLocal object for the
currently executing Thread.
|
void |
set(T value)
Sets the value associated with the ThreadLocal object for the currently
executing Thread.
|
public ThreadLocal()
protected T initialValue()
null
.
Often, this method is overridden to create the appropriate initial object
for the current thread's view of the ThreadLocal.public T get()
initialValue()
.public void set(T value)
initialValue()
from being
called if this is the first access to this ThreadLocal in this Thread.value
- the value to set this thread's view of the variable topublic void remove()
protected void finalize() throws Throwable
Object
java.lang.ref
package.
Virtual Machines are free to not call this method if
they can determine that it does nothing important; for
example, if your class extends Object and overrides
finalize to do simply super.finalize()
.
finalize() will be called by a Thread
that has no
locks on any Objects, and may be called concurrently.
There are no guarantees on the order in which multiple
objects are finalized. This means that finalize() is
usually unsuited for performing actions that must be
thread-safe, and that your implementation must be
use defensive programming if it is to always work.
If an Exception is thrown from finalize() during garbage collection, it will be patently ignored and the Object will still be destroyed.
It is allowed, although not typical, for user code to call finalize() directly. User invocation does not affect whether automatic invocation will occur. It is also permitted, although not recommended, for a finalize() method to "revive" an object by making it reachable from normal code again.
Unlike constructors, finalize() does not get called
for an object's superclass unless the implementation
specifically calls super.finalize()
.
The default implementation does nothing.
finalize
in class Object
Throwable
- permits a subclass to throw anything in an
overridden version; but the default throws nothingSystem.gc()
,
System.runFinalizersOnExit(boolean)
,
java.lang.ref