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java.lang.Objectjava.lang.ThreadLocal<T>public class ThreadLocal<T>extends Objectget() 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 Summary | |
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Method Summary | |
protected void |
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T |
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protected T |
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void |
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void |
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Methods inherited from class java.lang.Object | |
clone, equals, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait | |
public ThreadLocal()
Creates a ThreadLocal object without associating any value to it yet.
protected void finalize() throws Throwable
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. You would think that this means it eventually is called on every Object, but this is not necessarily the case. If execution terminates abnormally, garbage collection does not always happen. Thus you cannot rely on this method to always work. For finer control over garbage collection, use references from thejava.lang.refpackage.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
Threadthat 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.
- Throws:
Throwable- permits a subclass to throw anything in an overridden version; but the default throws nothing
- See Also:
System.gc(),System.runFinalizersOnExit(boolean),java.lang.ref
public T get()
Gets the value associated with the ThreadLocal object for the currently executing Thread. If this is the first time the current thread has called get(), and it has not already called set(), the value is obtained byinitialValue().
- Returns:
- the value of the variable in this thread
protected T initialValue()
Called once per thread on the first invocation of get(), if set() was not already called. The default implementation returnsnull. Often, this method is overridden to create the appropriate initial object for the current thread's view of the ThreadLocal.
- Returns:
- the initial value of the variable in this thread
public void remove()
Removes the value associated with the ThreadLocal object for the currently executing Thread.
- Since:
- 1.5
public void set(T value)
Sets the value associated with the ThreadLocal object for the currently executing Thread. This overrides any existing value associated with the current Thread and preventsinitialValue()from being called if this is the first access to this ThreadLocal in this Thread.
- Parameters:
value- the value to set this thread's view of the variable to