How can I pass a parameter to a Java Thread

Question

Can anyone suggest to me how I can pass a parameter to a thread   Also  how does it work for anonymous classes

User · Answer

I know that I m a few years late  but I came across this issue and took an unorthodox approach  I wanted to do it without making a new class  so this is what I came up with    int x   0  new Thread  new Runnable          int x       public void run                stuff with x and whatever else you want             public Runnable pass int x               this x   x             return this            pass x   start

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You can derive a class from Runnable  and during the construction  say  pass the parameter in   Then launch it using Thread start Runnable r    If you mean whilst the thread is running  then simply hold a reference to your derived object in the calling thread  and call the appropriate setter methods  synchronising where appropriate

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One further option  this approach lets you use the Runnable item like an asynchronous function call   If your task does not need to return a result  e g  it just performs some action you don t need to worry about how you pass back an  outcome    This pattern lets you reuse an item  where you need some kind of internal state   When not passing  parameter s  in the constructor care is needed to mediate the programs access to parameters   You may need more checks if your use-case involves different callers  etc   public class MyRunnable implements Runnable      private final Boolean PARAMETER LOCK    false    private X parameter     public MyRunnable X parameter         this parameter   parameter         public void setParameter  final X newParameter           boolean done   false        synchronize  PARAMETER LOCK                     if  null    parameter                             parameter   newParameter                done   true                            if    done                     throw new RuntimeException  MyRunnable - Parameter not cleared                      public void clearParameter           synchronize  PARAMETER LOCK                     parameter   null                  public void run            X localParameter         synchronize  PARAMETER LOCK                     localParameter   parameter                 if  null    localParameter                    clearParameter        -- could clear now  or later  or not at all              doSomeStuff  localParameter                      Thread t   new Thread new MyRunnable parameter       t start     If you need a result of processing  you will also need to coordinate completion of MyRunnable when the sub-task finishes   You could pass a call back or just wait on the Thread  t   etc

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In Java 8 you can use lambda expressions with the Concurrency API  amp  the ExecutorService as a higher level replacement for working with threads directly      newCachedThreadPool   Creates a thread pool that creates new threads   as needed  but will reuse previously constructed threads when they are   available  These pools will typically improve the performance of programs that execute many short-lived asynchronous tasks        private static final ExecutorService executor   Executors newCachedThreadPool         executor submit    - gt            myFunction myParam1  myParam2             See also executors javadocs

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This answer comes very late  but maybe someone will find it useful  It is about how to pass a parameter s  to a Runnable without even declaring named class  handy for inliners       String someValue    quot Just a demo  really    quot        new Thread new Runnable             private String myParam           public Runnable init String myParam                this myParam   myParam              return this                      Override         public void run                 System out println  quot This is called from another thread  quot                System out println this myParam                   init someValue   start     Of course you can postpone execution of start to some more convenient or appropriate moment  And it is up to you what will be the signature of init method  so it may take more and or different arguments  and of course even its name  but basically you get an idea  In fact there is also another way of passing a parameter to an anonymous class  with the use of the initializer blocks  Consider this      String someValue    quot Another demo  no serious thing    quot       int anotherValue   42       new Thread new Runnable             private String myParam          private int myOtherParam             instance initializer                       this myParam   someValue              this myOtherParam   anotherValue                      Override         public void run                 System out println  quot This comes from another thread  quot                System out println this myParam    quot    quot    this myOtherParam                    start     So all happens inside of the initializer block

User · Answer

You need to pass the parameter in the constructor to the Runnable object   public class MyRunnable implements Runnable       public MyRunnable Object parameter              store parameter for later user          public void run              and invoke it thus   Runnable r   new MyRunnable param value   new Thread r  start

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Either write a class that implements Runnable  and pass whatever you need in a suitably defined constructor  or write a class that extends Thread with a suitably defined constructor that calls super   with appropriate parameters

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via constructor of a Runnable or Thread class  class MyThread extends Thread        private String to       public MyThread String to            this to   to              Override     public void run             System out println  hello     to            public static void main String   args        new MyThread  world    start

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As of Java 8  you can use a lambda to capture parameters that are effectively final   For example   final String param1    First param   final int param2   2  new Thread    - gt           Do whatever you want here  param1 and param2 are in-scope      System out println param1       System out println param2      start

User · Answer

No you can t pass parameters to the run   method  The signature tells you that  it has no parameters   Probably the easiest way to do this would be to use a purpose-built object that takes a parameter in the constructor and stores it in a final variable    public class WorkingTask implements Runnable       private final Object toWorkWith       public WorkingTask Object workOnMe                toWorkWith   workOnMe             public void run                   do work                Thread t   new Thread new WorkingTask theData    t start      Once you do that - you have to be careful of the data integrity of the object you pass into the  WorkingTask   The data will now exist in two different threads so you have to make sure it is Thread Safe

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Create a local variable in your class that extends Thread or implements Runnable   public class Extractor extends Thread       public String webpage           public Extractor String w           webpage   w            public void setWebpage String l           webpage   l              Override     public void run       l is link         System out println webpage             public String toString            return  Page    webpage           This way  you can pass a variable when you run it   Extractor e   new Extractor  www google com    e start      The output    www google com

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Parameter passing via the start   and run   methods      Tester public static void main String    args  throws Exception       ThreadType2 t   new ThreadType2 new RunnableType2            public void run Object object                System out println  Parameter   object                    t start  the parameter          New class 1 of 2 public class ThreadType2       final private Thread thread      private Object objectIn   null      ThreadType2 final RunnableType2 runnableType2            thread   new Thread new Runnable                 public void run                     runnableType2 run objectIn                              public void start final Object object            this objectIn   object          thread start                 If you want to do things like setDaemon true        public Thread getThread             return thread              New class 2 of 2 public interface RunnableType2       public void run Object object

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Specially for Android  For callback purposes I usually implement my own generic Runnable with input parameter s    public interface Runnable lt TResult gt        void run TResult result       Usage is simple   myManager doCallbackOperation new Runnable lt MyResult gt           Override     public void run MyResult result               do something with the result             In manager   public void doCallbackOperation Runnable lt MyResult gt  runnable        new AsyncTask lt Void  Void  MyResult gt               Override         protected MyResult doInBackground Void    params                   do background operation             return new MyResult       return resulting object                     Override         protected void onPostExecute MyResult result                   execute runnable passing the result when operation has finished             runnable run result                   execute

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For Anonymous classes   In response to question edits here is how it works for Anonymous classes     final X parameter           the final is important    Thread t   new Thread new Runnable            p   parameter         public void run                                t start        Named classes   You have a class that extends Thread  or implements Runnable  and a constructor with the parameters you d like to pass  Then  when you create the new thread  you have to pass in the arguments  and then start the thread  something like this   Thread t   new MyThread args      t start      Runnable is a much better solution than Thread BTW  So I d prefer      public class MyRunnable implements Runnable         private X parameter        public MyRunnable X parameter             this parameter   parameter                 public void run                     Thread t   new Thread new MyRunnable parameter       t start      This answer is basically the same as this similar question  How to pass parameters to a Thread object

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You can either extend the Thread class or the Runnable class and provide parameters as you want  There are simple examples in the docs  I ll port them here    class PrimeThread extends Thread        long minPrime       PrimeThread long minPrime             this minPrime   minPrime               public void run                 compute primes larger than minPrime                             PrimeThread p   new PrimeThread 143    p start      class PrimeRun implements Runnable        long minPrime       PrimeRun long minPrime             this minPrime   minPrime               public void run                 compute primes larger than minPrime                              PrimeRun p   new PrimeRun 143    new Thread p  start

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To create a thread you normally create your own implementation of Runnable  Pass the parameters to the thread in the constructor of this class   class MyThread implements Runnable     private int a     private String b     private double c      public MyThread int a  String b  double c         this a   a        this b   b        this c   c           public void run          doSomething a  b  c

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There is a simple way of passing parameters into runnables  Code   public void Function final type variable        Runnable runnable   new Runnable             public void run                   Code adding here                         new Thread runnable  start

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When you create a thread  you need an instance of Runnable  The easiest way to pass in a parameter would be to pass it in as an argument to the constructor   public class MyRunnable implements Runnable        private volatile String myParam       public MyRunnable String myParam           this myParam   myParam                         public void run               do something with myParam here                       MyRunnable myRunnable   new myRunnable  Hello World    new Thread myRunnable  start      If you then want to change the parameter while the thread is running  you can simply add a setter method to your runnable class   public void setMyParam String value       this myParam   value      Once you have this  you can change the value of the parameter by calling like this   myRunnable setMyParam  Goodbye World      Of course  if you want to trigger an action when the parameter is changed  you will have to use locks  which makes things considerably more complex

[java] How can I pass a parameter to a Java Thread?

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