Java ThreadLocal
source from http://tutorials.jenkov.com/java-concurrency/threadlocal.html#inheritableThreadLocal
Java ThreadLocal
The ThreadLocal
class in Java enables you to create variables that can only be read and written by
the same thread. Thus, even if two threads are executing the same code, and the code has a reference to a
ThreadLocal
variable, then the two threads cannot see each other's ThreadLocal
variables.
Creating a ThreadLocal
Here is a code example that shows how to create a ThreadLocal
variable:
private ThreadLocal myThreadLocal = new ThreadLocal();
As you can see, you instantiate a new ThreadLocal
object. This only needs to be done once per thread. Even if different threads execute the same code which accesses a ThreadLococal
, each thread will see only its own ThreadLocal
instance. Even if two different threads set different values on the same ThreadLocal
object, they cannot see each other's values.
Accessing a ThreadLocal
Once a ThreadLocal
has been created you can set the value to be stored in it like this:
myThreadLocal.set("A thread local value");
You read the value stored in a ThreadLocal
like this:
String threadLocalValue = (String) myThreadLocal.get();
The get()
method returns an Object
and the set()
method takes an Object
as parameter.
Generic ThreadLocal
You can create a generic ThreadLocal
so that you do not have to typecast the value returned by get()
. Here is a generic ThreadLocal
example:
private ThreadLocal<String> myThreadLocal = new ThreadLocal<String>();
Now you can only store strings in the ThreadLocal
instance. Additionally, you do not need to typecast the value obtained from the ThreadLocal
:
myThreadLocal.set("Hello ThreadLocal"); String threadLocalValue = myThreadLocal.get();
Initial ThreadLocal Value
Since values set on a ThreadLocal
object only are visible to the thread who set the value, no thread can set an initial value on a ThreadLocal
using set()
which is visible to all threads.
Instead you can specify an initial value for a ThreadLocal
object by subclassing ThreadLocal
and overriding the initialValue()
method. Here is how that looks:
private ThreadLocal myThreadLocal = new ThreadLocal<String>() { @Override protected String initialValue() { return "This is the initial value"; } };
Now all threads will see the same initial value when calling get()
before having called set()
.
Full ThreadLocal Example
Here is a fully runnable Java ThreadLocal
example:
public class ThreadLocalExample { public static class MyRunnable implements Runnable { private ThreadLocal<Integer> threadLocal = new ThreadLocal<Integer>(); @Override public void run() { threadLocal.set( (int) (Math.random() * 100D) ); try { Thread.sleep(2000); } catch (InterruptedException e) { } System.out.println(threadLocal.get()); } } public static void main(String[] args) { MyRunnable sharedRunnableInstance = new MyRunnable(); Thread thread1 = new Thread(sharedRunnableInstance); Thread thread2 = new Thread(sharedRunnableInstance); thread1.start(); thread2.start(); thread1.join(); //wait for thread 1 to terminate thread2.join(); //wait for thread 2 to terminate } }
This example creates a single MyRunnable
instance which is passed to two different threads. Both threads execute the run()
method, and thus sets different values on the ThreadLocal
instance. If the access to the set()
call had been synchronized, and it had not been a ThreadLocal
object, the second thread would have overridden the value set by the first thread.
However, since it is a ThreadLocal
object then the two threads cannot see each other's values. Thus, they set and get different values.
InheritableThreadLocal
The InheritableThreadLocal
class is a subclass of ThreadLocal
. Instead of each thread having its own value inside a ThreadLocal
, the InheritableThreadLocal
grants access to values to a thread and all child threads created by that thread.
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