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  1. 1
    RichieHindle@email.null'

    RichieHindle

    The documentation says it very well: “each thread that accesses [a thread-local variable] (via its get or set method) has its own, independently initialized copy of the variable”.

    You use one when each thread must have its own copy of something. By default, data is shared between threads.

    Reply
  2. 2
    Dimos@email.null'

    Dimos

    ThreadLocal is useful, when you want to have some state that should not be shared amongst different threads, but it should be accessible from each thread during its whole lifetime.

    As an example, imagine a web application, where each request is served by a different thread. Imagine that for each request you need a piece of data multiple times, which is quite expensive to compute. However, that data might have changed for each incoming request, which means that you can’t use a plain cache. A simple, quick solution to this problem would be to have a ThreadLocal variable holding access to this data, so that you have to calculate it only once for each request. Of course, this problem can also be solved without the use of ThreadLocal, but I devised it for illustration purposes.

    That said, have in mind that ThreadLocals are essentially a form of global state. As a result, it has many other implications and should be used only after considering all the other possible solutions.

    Reply
  3. 3
    Ajay@email.null'

    Ajay

    ThreadLocal is a specially provisioned functionality by JVM to provide an isolated storage space for threads only. like the value of instance scoped variable are bound to a given instance of a class only. each object has its only values and they can not see each other value. so is the concept of ThreadLocal variables, they are local to the thread in the sense of object instances other thread except for the one which created it, can not see it. See Here

    import java.util.concurrent.atomic.AtomicInteger;
    import java.util.stream.IntStream;
    
    
    public class ThreadId {
    private static final AtomicInteger nextId = new AtomicInteger(1000);
    
    // Thread local variable containing each thread's ID
    private static final ThreadLocal<Integer> threadId = ThreadLocal.withInitial(() -> nextId.getAndIncrement());
    
    
    // Returns the current thread's unique ID, assigning it if necessary
    public static int get() {
        return threadId.get();
    }
    
    public static void main(String[] args) {
    
        new Thread(() -> IntStream.range(1, 3).forEach(i -> {
            System.out.println(Thread.currentThread().getName() + " >> " + new ThreadId().get());
        })).start();
    
        new Thread(() -> IntStream.range(1, 3).forEach(i -> {
            System.out.println(Thread.currentThread().getName() + " >> " + new ThreadId().get());
        })).start();
    
        new Thread(() -> IntStream.range(1, 3).forEach(i -> {
            System.out.println(Thread.currentThread().getName() + " >> " + new ThreadId().get());
        })).start();
    
    }
    }
    
    Reply
  4. 4
    infoj@email.null'

    infoj

    Two use cases where threadlocal variable can be used –
    1- When we have a requirement to associate state with a thread (e.g., a user ID or Transaction ID). That usually happens with a web application that every request going to a servlet has a unique transactionID associated with it.

    // This class will provide a thread local variable which
    // will provide a unique ID for each thread
    class ThreadId {
        // Atomic integer containing the next thread ID to be assigned
        private static final AtomicInteger nextId = new AtomicInteger(0);
    
        // Thread local variable containing each thread's ID
        private static final ThreadLocal<Integer> threadId =
            ThreadLocal.<Integer>withInitial(()-> {return nextId.getAndIncrement();});
    
        // Returns the current thread's unique ID, assigning it if necessary
        public static int get() {
            return threadId.get();
        }
    }
    

    Note that here the method withInitial is implemented using lambda expression.
    2- Another use case is when we want to have a thread safe instance and we don’t want to use synchronization as the performance cost with synchronization is more. One such case is when SimpleDateFormat is used. Since SimpleDateFormat is not thread safe so we have to provide mechanism to make it thread safe.

    public class ThreadLocalDemo1 implements Runnable {
        // threadlocal variable is created
        private static final ThreadLocal<SimpleDateFormat> dateFormat = new ThreadLocal<SimpleDateFormat>(){
            @Override
            protected SimpleDateFormat initialValue(){
                System.out.println("Initializing SimpleDateFormat for - " + Thread.currentThread().getName() );
                return new SimpleDateFormat("dd/MM/yyyy");
            }
        };
    
        public static void main(String[] args) {
            ThreadLocalDemo1 td = new ThreadLocalDemo1();
            // Two threads are created
            Thread t1 = new Thread(td, "Thread-1");
            Thread t2 = new Thread(td, "Thread-2");
            t1.start();
            t2.start();
        }
    
        @Override
        public void run() {
            System.out.println("Thread run execution started for " + Thread.currentThread().getName());
            System.out.println("Date formatter pattern is  " + dateFormat.get().toPattern());
            System.out.println("Formatted date is " + dateFormat.get().format(new Date()));
        } 
    
    }
    
    Reply
  5. 5
    znlyj@email.null'

    znlyj

    Webapp server may keep a thread pool, and a ThreadLocal var should be removed before response to the client, thus current thread may be reused by next request.

    Reply
  6. 6
    Limestone@email.null'

    Limestone

    when?

    When an object is not thread-safe, instead of synchronization which hampers the scalability, give one object to every thread and keep it thread scope, which is ThreadLocal. One of most often used but not thread-safe objects are database Connection and JMSConnection.

    How ?

    One example is Spring framework uses ThreadLocal heavily for managing transactions behind the scenes by keeping these connection objects in ThreadLocal variables. At high level, when a transaction is started it gets the connection ( and disables the auto commit ) and keeps it in ThreadLocal. on further db calls it uses same connection to communicate with db. At the end, it takes the connection from ThreadLocal and commits ( or rollback ) the transaction and releases the connection.

    I think log4j also uses ThreadLocal for maintaining MDC.

    Reply
  7. 7
    Kanagavelu

    Kanagavelu Sugumar

    ThreadLocal will ensure accessing the mutable object by the multiple
    threads in the non synchronized method is synchronized, means making
    the mutable object to be immutable within the method.

    This
    is achieved by giving new instance of mutable object for each thread
    try accessing it. So It is local copy to the each thread. This is some
    hack on making instance variable in a method to be accessed like a
    local variable. As you aware method local variable is only available
    to the thread, one difference is; method local variables will not
    available to the thread once method execution is over where as mutable
    object shared with threadlocal will be available across multiple
    methods till we clean it up.

    By Definition:

    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.

    Each Thread in java contains ThreadLocalMap in it.
    Where

    Key = One ThreadLocal object shared across threads.
    value = Mutable object which has to be used synchronously, this will be instantiated for each thread.
    

    Achieving the ThreadLocal:

    Now create a wrapper class for ThreadLocal which is going to hold the mutable object like below (with or without initialValue()).
    Now getter and setter of this wrapper will work on threadlocal instance instead of mutable object.

    If getter() of threadlocal didn’t find any value with in the threadlocalmap of the Thread; then it will invoke the initialValue() to get its private copy with respect to the thread.

    class SimpleDateFormatInstancePerThread {
    
        private static final ThreadLocal<SimpleDateFormat> dateFormatHolder = new ThreadLocal<SimpleDateFormat>() {
    
            @Override
            protected SimpleDateFormat initialValue() {
                SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd") {
                    UUID id = UUID.randomUUID();
                    @Override
                    public String toString() {
                        return id.toString();
                    };
                };
                System.out.println("Creating SimpleDateFormat instance " + dateFormat +" for Thread : " + Thread.currentThread().getName());
                return dateFormat;
            }
        };
    
        /*
         * Every time there is a call for DateFormat, ThreadLocal will return calling
         * Thread's copy of SimpleDateFormat
         */
        public static DateFormat getDateFormatter() {
            return dateFormatHolder.get();
        }
    
        public static void cleanup() {
            dateFormatHolder.remove();
        }
    }
    

    Now wrapper.getDateFormatter() will call threadlocal.get() and that will check the currentThread.threadLocalMap contains this (threadlocal) instance.
    If yes return the value (SimpleDateFormat) for corresponding threadlocal instance
    else add the map with this threadlocal instance, initialValue().

    Herewith thread safety achieved on this mutable class; by each thread is working with its own mutable instance but with same ThreadLocal instance. Means All the thread will share the same ThreadLocal instance as key, but different SimpleDateFormat instance as value.

    https://github.com/skanagavelu/yt.tech/blob/master/src/ThreadLocalTest.java

    Reply
  8. 8
    Rakesh

    Rakesh Chauhan

    There is very good example in book Java Concurrency in Practice. Where he explains how Thread confinement is one of the simplest ways to achieve thread safety and Thread Local is more formal means of maintaining thread confinement. In the end he also explain how people can abuse it by using it as global variables.

    I have copied the text from the mentioned book but code 3.10 is missing as it is not much important to understand where Thread Local should be use.

    Thread-local variables are often used to prevent sharing in designs based on mutable Singletons or global variables. For example, a single-threaded application might maintain a global database connection that is initialized at startup to avoid having to pass a Connection to every method. Since JDBC connections may not be thread-safe, a multithreaded application that uses a global connection without additional coordination is not thread-safe either. By using a ThreadLocal to store the JDBC connection, as in ConnectionHolder in Listing 3.10, each thread will have its own connection.

    ThreadLocal is widely used in implementing application frameworks. For example, J2EE containers associate a transaction context with an executing thread for the duration of an EJB call. This is easily implemented using a static Thread-Local holding the transaction context: when framework code needs to determine what transaction is currently running, it fetches the transaction context from this ThreadLocal. This is convenient in that it reduces the need to pass execution context information into every method, but couples any code that uses this mechanism to the framework.

    It is easy to abuse ThreadLocal by treating its thread confinement property as a license to use global variables or as a means of creating “hidden” method arguments. Like global variables, thread-local variables can detract from reusability and introduce hidden couplings among classes, and should therefore be used with care.

    Reply
  9. 9
    Abhijit

    Abhijit Gaikwad

    1. ThreadLocal in Java had been introduced on JDK 1.2 but was later generified in JDK 1.5 to introduce type safety on ThreadLocal variable.

    2. ThreadLocal can be associated with Thread scope, all the code which is executed by Thread has access to ThreadLocal variables but two thread can not see each others ThreadLocal variable.

    3. Each thread holds an exclusive copy of ThreadLocal variable which becomes eligible to Garbage collection after thread finished or died, normally or due to any Exception, Given those ThreadLocal variable doesn’t have any other live references.

    4. ThreadLocal variables in Java are generally private static fields in Classes and maintain its state inside Thread.

    Read more: http://javarevisited.blogspot.com/2012/05/how-to-use-threadlocal-in-java-benefits.html#ixzz2XltgbHTK

    Reply
  10. 10
    Jeff

    Jeff Richley

    You have to be very careful with the ThreadLocal pattern. There are some major down sides like Phil mentioned, but one that wasn’t mentioned is to make sure that the code that sets up the ThreadLocal context isn’t “re-entrant.”

    Bad things can happen when the code that sets the information gets run a second or third time because information on your thread can start to mutate when you didn’t expect it. So take care to make sure the ThreadLocal information hasn’t been set before you set it again.

    Reply
  11. 11
    user100464@email.null'

    user100464

    In Java, if you have a datum that can vary per-thread, your choices are to pass that datum around to every method that needs (or may need) it, or to associate the datum with the thread. Passing the datum around everywhere may be workable if all your methods already need to pass around a common “context” variable.

    If that’s not the case, you may not want to clutter up your method signatures with an additional parameter. In a non-threaded world, you could solve the problem with the Java equivalent of a global variable. In a threaded word, the equivalent of a global variable is a thread-local variable.

    Reply
  12. 12
    overthink@email.null'

    overthink

    One possible (and common) use is when you have some object that is not thread-safe, but you want to avoid synchronizing access to that object (I’m looking at you, SimpleDateFormat). Instead, give each thread its own instance of the object.

    For example:

    public class Foo
    {
        // SimpleDateFormat is not thread-safe, so give one to each thread
        private static final ThreadLocal<SimpleDateFormat> formatter = new ThreadLocal<SimpleDateFormat>(){
            @Override
            protected SimpleDateFormat initialValue()
            {
                return new SimpleDateFormat("yyyyMMdd HHmm");
            }
        };
    
        public String formatIt(Date date)
        {
            return formatter.get().format(date);
        }
    }
    

    Documentation.

    Reply
  13. 13
    Robin@email.null'

    Robin

    As was mentioned by @unknown (google), it’s usage is to define a global variable in which the value referenced can be unique in each thread. It’s usages typically entails storing some sort of contextual information that is linked to the current thread of execution.

    We use it in a Java EE environment to pass user identity to classes that are not Java EE aware (don’t have access to HttpSession, or the EJB SessionContext). This way the code, which makes usage of identity for security based operations, can access the identity from anywhere, without having to explicitly pass it in every method call.

    The request/response cycle of operations in most Java EE calls makes this type of usage easy since it gives well defined entry and exit points to set and unset the ThreadLocal.

    Reply
  14. 14
    Esko

    Esko Luontola

    Many frameworks use ThreadLocals to maintain some context related to the current thread. For example when the current transaction is stored in a ThreadLocal, you don’t need to pass it as a parameter through every method call, in case someone down the stack needs access to it. Web applications might store information about the current request and session in a ThreadLocal, so that the application has easy access to them. With Guice you can use ThreadLocals when implementing custom scopes for the injected objects (Guice’s default servlet scopes most probably use them as well).

    ThreadLocals are one sort of global variables (although slightly less evil because they are restricted to one thread), so you should be careful when using them to avoid unwanted side-effects and memory leaks. Design your APIs so that the ThreadLocal values will always be automatically cleared when they are not anymore needed and that incorrect use of the API won’t be possible (for example like this). ThreadLocals can be used to make the code cleaner, and in some rare cases they are the only way to make something work (my current project had two such cases; they are documented here under “Static Fields and Global Variables”).

    Reply
  15. 15
    Neil

    Neil Coffey

    Essentially, when you need a variable’s value to depend on the current thread and it isn’t convenient for you to attach the value to the thread in some other way (for example, subclassing thread).

    A typical case is where some other framework has created the thread that your code is running in, e.g. a servlet container, or where it just makes more sense to use ThreadLocal because your variable is then “in its logical place” (rather than a variable hanging from a Thread subclass or in some other hash map).

    On my web site, I have some further discussion and examples of when to use ThreadLocal that may also be of interest.

    Some people advocate using ThreadLocal as a way to attach a “thread ID” to each thread in certain concurrent algorithms where you need a thread number (see e.g. Herlihy & Shavit). In such cases, check that you’re really getting a benefit!

    Reply
  16. 16
    Colselaw@email.null'

    Colselaw

    Nothing really new here, but I discovered today that ThreadLocal is very useful when using Bean Validation in a web application. Validation messages are localized, but by default use Locale.getDefault(). You can configure the Validator with a different MessageInterpolator, but there’s no way to specify the Locale when you call validate. So you could create a static ThreadLocal<Locale> (or better yet, a general container with other things you might need to be ThreadLocal and then have your custom MessageInterpolator pick the Locale from that. Next step is to write a ServletFilter which uses a session value or request.getLocale() to pick the locale and store it in your ThreadLocal reference.

    Reply
  17. 17
    Phil

    Phil M

    Since a ThreadLocal is a reference to data within a given Thread, you can end up with classloading leaks when using ThreadLocals in application servers which use thread pools. You need to be very careful about cleaning up any ThreadLocals you get() or set() by using the ThreadLocal‘s remove() method.

    If you do not clean up when you’re done, any references it holds to classes loaded as part of a deployed webapp will remain in the permanent heap and will never get garbage collected. Redeploying/undeploying the webapp will not clean up each Thread‘s reference to your webapp’s class(es) since the Thread is not something owned by your webapp. Each successive deployment will create a new instance of the class which will never be garbage collected.

    You will end up with out of memory exceptions due to java.lang.OutOfMemoryError: PermGen space and after some googling will probably just increase -XX:MaxPermSize instead of fixing the bug.

    If you do end up experiencing these problems, you can determine which thread and class is retaining these references by using Eclipse’s Memory Analyzer and/or by following Frank Kieviet’s guide and followup.

    Update: Re-discovered Alex Vasseur’s blog entry that helped me track down some ThreadLocal issues I was having.

    Reply
  18. 18
    Ian

    Ian Ringrose

    Caching, sometime you have to calculate the same value lots of time so by storing the last set of inputs to a method and the result you can speed the code up. By using Thread Local Storage you avoid having to think about locking.

    Reply

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