Java ThreadLocal Example

Java ThreadLocal is used to create thread local variables. We know that all threads of an Object share it’s variables, so the variable is not thread safe. We can use synchronization for thread safety but if we want to avoid synchronization, we can use ThreadLocal variables.

Java ThreadLocal

Every thread has it’s own ThreadLocal variable and they can use it’s get() and set() methods to get the default value or change it’s value local to Thread.

ThreadLocal instances are typically private static fields in classes that wish to associate state with a thread.

Java ThreadLocal Example

Here is a small example showing use of ThreadLocal in java program and proving that every thread has it’s own copy of ThreadLocal variable.

ThreadLocalExample.java

Output of the above java ThreadLocal example program is:

As you can see from the output that Thread-0 has changed the value of formatter but still thread-2 default formatter is same as the initialized value. You can see the same pattern for other threads too.

Update: ThreadLocal class is extend in Java 8 with a new method withInitial() that takes Supplier functional interface as argument. So we can use lambda expressions to easily create the ThreadLocal instance. For example, above formatter ThreadLocal variable can be defined in one line as below:

If you are new to Java 8 features, please check out Java 8 Features and Java 8 Functional Interfaces.

That’s all for ThreadLocal in java programming.

Reference: API Doc

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