How fast is ThreadLocal?

Summary. A thread local is around 10-20x that of the heap read.

Why is ThreadLocal bad?

It’s a terrible practice: a) it’s a pool of one or more global variables and global variables in any language are bad practice (there’s a whole set of problems associated with global variables – search it on the net) b) it may lead to memory leaks, in any j2ee container than manages its threads, if you don’t handle it …

Is ThreadLocal safe?

1 Answer. It’s safe because getMap returns the map for the given (i.e. current) thread. No other thread is going to be messing with that.

What is ThreadLocal in Java why is it used?

The Java ThreadLocal class 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 the same ThreadLocal variable, the two threads cannot see each other’s ThreadLocal variables.

Can ThreadLocal cause memory leak?

Memory leak is caused when ThreadLocal is always existing. If ThreadLocal object could be GC, it will not cause memory leak. Because the entry in ThreadLocalMap extends WeakReference, the entry will be GC after ThreadLocal object is GC.

When should I use ThreadLocal?

Most common use of thread local is when you have some object that is not thread-safe, but you want to avoid synchronizing access to that object using synchronized keyword/block. Instead, give each thread its own instance of the object to work with.

When should ThreadLocal be removed?

Because ThreadLocal has Map of currentThread and value , Now if you don’t remove the value in the thread which was using it then it will create a memory leak. You should always call remove because ThreadLocal class puts values from the Thread Class defined by ThreadLocal.

Can ThreadLocal be static?

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.

How do you clear a local variable in a thread?

There is no way to cleanup ThreadLocal values except from within the thread that put them in there in the first place (or when the thread is garbage collected – not the case with worker threads).

Does ThreadLocal have to be static?

ThreadLocal s should be stored in static variables to avoid memory leaks. If a ThreadLocal is stored in an instance (non-static) variable, there will be M \* N instances of the ThreadLocal value where M is the number of threads, and N is the number of instances of the containing class.

What is thread local in Java?

ThreadLocal in Java is another way to achieve thread-safety apart from writing immutable classes. Thread local can be considered as a scope of access like session scope or request scope. In thread local, you can set any object and this object will be local and global to the specific thread which is accessing this object.

Is it safe to use threadlocals with threadpools in Java?

This is a reasonable way of achieving thread-safety in Java. However, we should be extra careful when we’re using ThreadLocals and thread pools together. In order to better understand the possible caveat, let’s consider the following scenario:

What is the use of ThreadLocal variable?

It enables you to create variables that can only be read and write by the same thread. If two threads are executing the same code and that code has a reference to a ThreadLocal variable then the two threads can’t see the local variable of each other. The variable of ThreadLocal can be created using the following code:

How to set the initial value of a ThreadLocal object?

No thread can set an initial value on a ThreadLocal using set () method which is visible to all threads. Instead you can specify an initial value for a ThreadLocal object by subclassing ThreadLocal and overriding the initialValue (). Now all threads can see the same initial value when calling the get () method before having called set () method.