How to catch an Exception from a thread

Question

I have Java main class  in the class  I start a new thread  in the main  it waits until the thread dies  At some moment  I throw a runtime exception from the thread  but I can t catch the exception thrown from the thread in the main class    Here is the code   public class Test extends Thread     public static void main String   args  throws InterruptedException         Test t   new Test         try             t start          t join              catch RuntimeException e              System out println     RuntimeException from main               System out println  Main stoped            Override   public void run           try             while true                  System out println     Started             sleep 2000            throw new RuntimeException  exception from thread                      catch  RuntimeException e              System out println     RuntimeException from thread           throw e             catch  InterruptedException e                       Anybody knows why

User · Answer

If you implement Thread UncaughtExceptionHandler in class that starts the Threads  you can set and then rethrow the exception   public final class ThreadStarter implements Thread UncaughtExceptionHandler   private volatile Throwable initException       public void doSomeInit            Thread t   new Thread                 Override             public void run                   throw new RuntimeException  UNCAUGHT                                     t setUncaughtExceptionHandler this            t start            t join             if  initException    null               throw new RuntimeException initException                          Override     public void uncaughtException Thread t  Throwable e            initException    e                 Which causes the following output   Exception in thread  main  java lang RuntimeException  java lang RuntimeException  UNCAUGHT     at com gs gss ccsp enrichments ThreadStarter doSomeInit ThreadStarter java 24      at com gs gss ccsp enrichments ThreadStarter main ThreadStarter java 38      at sun reflect NativeMethodAccessorImpl invoke0 Native Method      at sun reflect NativeMethodAccessorImpl invoke NativeMethodAccessorImpl java 39      at sun reflect DelegatingMethodAccessorImpl invoke DelegatingMethodAccessorImpl java 25      at java lang reflect Method invoke Method java 597      at com intellij rt execution application AppMain main AppMain java 120  Caused by  java lang RuntimeException  UNCAUGHT     at com gs gss ccsp enrichments ThreadStarter 1 run ThreadStarter java 15

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Exception handling in Thread   By default run   method doesn   t throw any exception  so all checked exceptions inside the run method has to be caught and handled there only and for runtime exceptions we can use UncaughtExceptionHandler  UncaughtExceptionHandler is an interface provided by Java to handle exceptions in a Thread run method  So we can implement this interface and set back our implementing class back to Thread object using setUncaughtExceptionHandler   method  But this handler has to be set before we call start   on the tread   if we don   t set uncaughtExceptionHandler then the Threads ThreadGroup acts as a handler    public class FirstThread extends Thread    int count   0    Override public void run         while  true            System out println  FirstThread doing something urgent  count                        count              throw new RuntimeException              public static void main String   args        FirstThread t1   new FirstThread        t1 setUncaughtExceptionHandler new UncaughtExceptionHandler             public void uncaughtException Thread t  Throwable e                System out printf  Exception thrown by  s with id    d                       t getName    t getId                 System out println   n  e getClass                           t1 start          Nice explanation given at  http   coder2design com thread-creation  exceptions

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AtomicReference is also a solution to pass the error to the main thread  Is same approach like the one of Dan Cruz    AtomicReference lt Throwable gt  errorReference   new AtomicReference lt  gt          Thread thread   new Thread             public void run                 throw new RuntimeException  TEST EXCEPTION                          thread setUncaughtExceptionHandler  th  ex  - gt            errorReference set ex               thread start        thread join        Throwable newThreadError  errorReference get        if  newThreadError   null            throw newThreadError            The only change is that instead of creating a volatile variable you can use AtomicReference which did same thing behind the scenes

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Most likely     you don t need to pass the exception from one thread to another  if you want to handle an exception  just do it in the thread which threw it  your main thread doesn t need to wait from the background thread in this example  which actually means you don t need a background thread at all    However  lets assume you do need to handle an exception from a child thread another   I would use an ExecutorService like this   ExecutorService executor   Executors newSingleThreadExecutor    Future lt Void gt  future   executor submit new Callable lt Void gt           Override     public Void call   throws Exception           System out println     Started            Thread sleep 2000           throw new IllegalStateException  exception from thread              try       future get       raises ExecutionException for any uncaught exception in child   catch  ExecutionException e        System out println     RuntimeException from thread         e getCause   printStackTrace System out     executor shutdown    System out println     Main stopped      prints     Started    RuntimeException from thread  java lang IllegalStateException  exception from thread     at Main 1 call Main java 11      at Main 1 call Main java 6      at java util concurrent FutureTask Sync innerRun FutureTask java 303      at java util concurrent FutureTask run FutureTask java 138      at java util concurrent ThreadPoolExecutor Worker runTask ThreadPoolExecutor java 886      at java util concurrent ThreadPoolExecutor Worker run ThreadPoolExecutor java 908      at java lang Thread run Thread java 662     Main stopped

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It is almost always wrong to extend Thread  I cannot state this strongly enough   Multithreading Rule  1  Extending Thread is wrong    If you implement Runnable instead you will see your expected behaviour   public class Test implements Runnable      public static void main String   args        Test t   new Test        try         new Thread t  start          catch  RuntimeException e          System out println     RuntimeException from main               System out println  Main stoped             Override   public void run         try         while  true            System out println     Started             Thread sleep 2000            throw new RuntimeException  exception from thread                  catch  RuntimeException e          System out println     RuntimeException from thread          throw e        catch  InterruptedException e                   produces   Main stoped    Started    RuntimeException from threadException in thread  Thread-0  java lang RuntimeException  exception from thread     at Test run Test java 23      at java lang Thread run Thread java 619      unless you want to change the way your application uses threads  which in 99 9  of cases you don t  If you think you are in the 0 1  of cases  please see rule  1

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You cannot do this  since it doesn t really make sense  If you hadn t called t join   then you main thread could be anywhere in the code when the t thread throws an exception

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Use a Thread UncaughtExceptionHandler  Thread UncaughtExceptionHandler h   new Thread UncaughtExceptionHandler          Override     public void uncaughtException Thread th  Throwable ex            System out println  quot Uncaught exception   quot    ex            Thread t   new Thread          Override     public void run             System out println  quot Sleeping     quot            try               Thread sleep 1000             catch  InterruptedException e                System out println  quot Interrupted  quot                      System out println  quot Throwing exception     quot            throw new RuntimeException             t setUncaughtExceptionHandler h   t start

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That s because exceptions are local to a thread  and your main thread doesn t actually see the run method  I suggest you read more about how threading works  but to quickly summarize  your call to start starts up a different thread  totally unrelated to your main thread  The call to join simply waits for it to be done  An exception that is thrown in a thread and never caught terminates it  which is why join returns on your main thread  but the exception itself is lost   If you want to be aware of these uncaught exceptions you can try this   Thread setDefaultUncaughtExceptionHandler new Thread UncaughtExceptionHandler          Override     public void uncaughtException Thread t  Throwable e            System out println  Caught     e               More information about uncaught exception handling can be found here

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Currently you are catching only RuntimeException  a sub class of Exception  But your application may throw other sub-classes of Exception  Catch generic Exception in addition to RuntimeException  Since many of things have been changed on Threading front  use advanced java API    Prefer advance java util concurrent API for multi-threading like ExecutorService or ThreadPoolExecutor    You can customize your ThreadPoolExecutor to handle exceptions   Example from oracle documentation page   Override   protected void afterExecute Runnable r                              Throwable t       Method invoked upon completion of execution of the given Runnable  This method is invoked by the thread that executed the task  If non-null  the Throwable is the uncaught RuntimeException or Error that caused execution to terminate abruptly    Example code   class ExtendedExecutor extends ThreadPoolExecutor                protected void afterExecute Runnable r  Throwable t         super afterExecute r  t        if  t    null  amp  amp  r instanceof Future lt   gt            try            Object result     Future lt   gt   r  get             catch  CancellationException ce               t   ce           catch  ExecutionException ee               t   ee getCause             catch  InterruptedException ie               Thread currentThread   interrupt       ignore reset                      if  t    null         System out println t             Usage   ExtendedExecutor service   new ExtendedExecutor      I have added one constructor on top of above code as    public ExtendedExecutor             super 1 5 60 TimeUnit SECONDS new ArrayBlockingQueue lt Runnable gt  100           You can change this constructor to suit your requirement on number of threads    ExtendedExecutor service   new ExtendedExecutor    service submit  lt your Callable or Runnable implementation gt

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Also from Java 8 you can write Dan Cruz answer as   Thread t   new Thread   - gt               System out println  Sleeping                    try                   Thread sleep 1000                 catch  InterruptedException e                    System out println  Interrupted                               System out println  Throwing exception                    throw new RuntimeException          t setUncaughtExceptionHandler  th  ex - gt  log String format  Exception in thread  d id   s   th getId    ex     t start

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My solution with RxJava    Test expectedExceptions   TestException class  public void testGetNonexistentEntry   throws Exception          using this to work around the limitation where the errors in onError  in subscribe method         cannot be thrown out to the main thread     AtomicReference lt Exception gt  ex   new AtomicReference lt  gt         URI id   getRandomUri        canonicalMedia setId id        client get id toString             subscribe              m - gt                  fail  Should not be successful                e - gt                  ex set new TestException           for int i   0  i  lt  5    i                if ex get      null              throw ex get            else             Thread sleep 1000             Assert fail  Cannot find the exception to throw

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Use Callable instead of Thread  then you can call Future get   which throws any exception that the Callable threw

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For those who needs to stop all Threads running and re-run all of them when any one of them is stopped on an Exception    Override public void onApplicationEvent ContextRefreshedEvent event             could be any function      getStockHistory         public void getStockHistory              fill a list of symbol to be scrapped      List lt String gt  symbolListNYSE   stockEntityRepository       findByExchangeShortNameOnlySymbol ContextRefreshExecutor NYSE         storeSymbolList symbolListNYSE  ContextRefreshExecutor NYSE        private void storeSymbolList List lt String gt  symbolList  String exchange         int total   symbolList size            I create a list of Thread      List lt Thread gt  listThread   new ArrayList lt Thread gt             For each 1000 element of my scrapping ticker list I create a new Thread     for  int i   0  i  lt   total  i    1000            int l   i           Thread t1   new Thread                  public void run                         just a service that store in DB my ticker list                 storingService getAndStoreStockPrice symbolList  l  1000                   MULTIPLE STOCK FILL  exchange                                   Thread UncaughtExceptionHandler h   new Thread UncaughtExceptionHandler                 public void uncaughtException Thread thread  Throwable exception                        stop thread if still running                 thread interrupt                        go over every thread running and stop every one of them                 listThread stream   forEach tread - gt  tread interrupt                         relaunch all the Thread via the main function                 getStockHistory                                      t1 start            t1 setUncaughtExceptionHandler h            listThread add t1               To sum up    You have a main function that create multiple thread  each of them has UncaughtExceptionHandler which is trigger by any Exception inside of a thread  You add every Thread to a List  If a UncaughtExceptionHandler is trigger it will loop through the List  stop every Thread and relaunch the main function recreation all the Thread

User · Answer

I faced the same issue     little work around  only for implementation not anonymous objects       we can declare the class level exception object as null     then initialize it inside the catch block for run method     if there was error in run method this variable wont be null    we can then have null check for this particular variable and if its not null then there was exception inside the thread execution      class TestClass implements Runnable      private Exception ex            Override         public void run                 try                    business code                 catch Exception e                      ex e                                      public void checkForException   throws Exception               if  ex   null                    throw ex                                   call checkForException   after join

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Please take a look at Thread UncaughtExceptionHandler  Better  alternative  way is to use Callable and Future to get the same result

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Did you play around with setDefaultUncaughtExceptionHandler   and the alike methods of the Thread class  From the API   By setting the default uncaught exception handler  an application can change the way in which uncaught exceptions are handled  such as logging to a specific device  or file  for those threads that would already accept whatever  default  behavior the system provided    You might find the answer to your problem there    good luck   -

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This explains the state transition of threads dependening on whether an exceptions occured or not     Source   http   www-public imtbs-tsp eu  gibson Teaching CSC7322 L8-ExceptionsAndThreads pdf

[java] How to catch an Exception from a thread

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