How do you implement a re-try-catch

Question

Try-catch is meant to help in the exception handling  This means somehow that it will help our system to be more robust  try to recover from an unexpected event    We suspect something might happen when executing and instruction  sending a message   so it gets enclosed in the try  If that something nearly unexpected happens  we can do something  we write the catch  I don t think we called to just log the exception  I thing the catch block is meant to give us the opportunity of recovering from the error   Now  let s say we recover from the error because we could fix what was wrong  It could be super nice to do a re-try   try  some instruction      catch  NearlyUnexpectedException e      fix the problem       retry      This would quickly fall in the eternal loop  but let s say that the fix the problem returns true  then we retry  Given that there is no such thing in Java  how would YOU solve this problem  What would be your best design code for solving this   This is like a philosophical question  given that I already know what I m asking for is not directly supported by Java

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You can use https   github com bnsd55 RetryCatch  Example   RetryCatch retryCatchSyncRunnable   new RetryCatch            retryCatchSyncRunnable                    For infinite retry times  just remove this row                  retryCount 3                     For retrying on all exceptions  just remove this row                  retryOn ArithmeticException class  IndexOutOfBoundsException class                   onSuccess    - gt  System out println  Success  There is no result because this is a runnable                      onRetry  retryCount  e  - gt  System out println  Retry count      retryCount      Exception message      e getMessage                      onFailure e - gt  System out println  Failure  Exception message      e getMessage                      run new ExampleRunnable       Instead of new ExampleRunnable   you can pass your own anonymous function

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If not all exceptions warrant a retry  only some  And if at least one try has to be made  Here is an alternative utility method   void runWithRetry Runnable runnable  Class lt Exception gt  exClass  int maxRetries            Exception err   null          do               maxRetries--              try                   runnable run                    err   null                catch  Exception e                    if exClass isAssignableFrom e getClass                          err   e                   else                       throw e                                            while  err    null  amp  amp  maxRetries  gt  0            if  err    null                throw err                    Usage       runWithRetry    - gt              do something        TimeoutException class  5

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You need to enclose your try-catch inside a while loop like this  -  int count   0  int maxTries   3  while true        try              Some Code            break out of loop  or return  on success       catch  SomeException e               handle exception         if    count    maxTries  throw e            I have taken count and maxTries to avoid running into an infinite loop  in case the exception keeps on occurring in your try block

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Most of these answers are essentially the same  Mine is also  but this is the form I like  boolean completed   false  Throwable lastException   null  for  int tryCount 0  tryCount  lt  config MAX SOME OPERATION RETRIES  tryCount          try           completed   some operation            break            catch  UnlikelyException e            lastException   e          fix the problem            if   completed        reportError lastException

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following is my solution with very simple approach                  while  true                        try                               Statement what may cause an error                          break                        catch  Exception e

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This is an old question but a solution is still relevant  Here is my generic solution in Java 8 without using any third party library   public interface RetryConsumer lt T gt        T evaluate   throws Throwable    public interface RetryPredicate lt T gt        boolean shouldRetry T t     public class RetryOperation lt T gt        private RetryConsumer lt T gt  retryConsumer      private int noOfRetry      private int delayInterval      private TimeUnit timeUnit      private RetryPredicate lt T gt  retryPredicate      private List lt Class lt   extends Throwable gt  gt  exceptionList       public static class OperationBuilder lt T gt            private RetryConsumer lt T gt  iRetryConsumer          private int iNoOfRetry          private int iDelayInterval          private TimeUnit iTimeUnit          private RetryPredicate lt T gt  iRetryPredicate          private Class lt   extends Throwable gt    exceptionClasses           private OperationBuilder                        public OperationBuilder lt T gt  retryConsumer final RetryConsumer lt T gt  retryConsumer                this iRetryConsumer   retryConsumer              return this                     public OperationBuilder lt T gt  noOfRetry final int noOfRetry                this iNoOfRetry   noOfRetry              return this                     public OperationBuilder lt T gt  delayInterval final int delayInterval  final TimeUnit timeUnit                this iDelayInterval   delayInterval              this iTimeUnit   timeUnit              return this                     public OperationBuilder lt T gt  retryPredicate final RetryPredicate lt T gt  retryPredicate                this iRetryPredicate   retryPredicate              return this                      SafeVarargs         public final OperationBuilder lt T gt  retryOn final Class lt   extends Throwable gt     exceptionClasses                this exceptionClasses   exceptionClasses              return this                     public RetryOperation lt T gt  build                 if  Objects isNull iRetryConsumer                     throw new RuntimeException    retryConsumer RetryConsumer lt T gt   not set                               List lt Class lt   extends Throwable gt  gt  exceptionList   new ArrayList lt  gt                 if  Objects nonNull exceptionClasses   amp  amp  exceptionClasses length  gt  0                    exceptionList   Arrays asList exceptionClasses                             iNoOfRetry   iNoOfRetry    0   1   0              iTimeUnit   Objects isNull iTimeUnit    TimeUnit MILLISECONDS   iTimeUnit              return new RetryOperation lt  gt  iRetryConsumer  iNoOfRetry  iDelayInterval  iTimeUnit  iRetryPredicate  exceptionList                        public static  lt T gt  OperationBuilder lt T gt  newBuilder             return new OperationBuilder lt  gt                private RetryOperation RetryConsumer lt T gt  retryConsumer  int noOfRetry  int delayInterval  TimeUnit timeUnit                             RetryPredicate lt T gt  retryPredicate  List lt Class lt   extends Throwable gt  gt  exceptionList            this retryConsumer   retryConsumer          this noOfRetry   noOfRetry          this delayInterval   delayInterval          this timeUnit   timeUnit          this retryPredicate   retryPredicate          this exceptionList   exceptionList             public T retry   throws Throwable           T result   null          int retries   0          while  retries  lt  noOfRetry                try                   result   retryConsumer evaluate                    if  Objects nonNull retryPredicate                         boolean shouldItRetry   retryPredicate shouldRetry result                       if  shouldItRetry                            retries   increaseRetryCountAndSleep retries                         else                           return result                                          else                          no retry condition defined  no exception thrown  This is the desired result                      return result                                  catch  Throwable e                    retries   handleException retries  e                                   return result             private int handleException int retries  Throwable e  throws Throwable           if  exceptionList contains e getClass        exceptionList isEmpty                      exception is excepted  continue retry              retries   increaseRetryCountAndSleep retries               if  retries    noOfRetry                       evaluation is throwing exception  no more retry left  Throw it                  throw e                          else                  unexpected exception  no retry required  Throw it              throw e                    return retries             private int increaseRetryCountAndSleep int retries            retries            if  retries  lt  noOfRetry  amp  amp  delayInterval  gt  0                try                   timeUnit sleep delayInterval                 catch  InterruptedException ignore                    Thread currentThread   interrupt                                    return retries            Let s have a test case like    Test public void withPredicateAndException         AtomicInteger integer   new AtomicInteger        try           Integer result   RetryOperation  lt Integer gt newBuilder                    retryConsumer    - gt                        int i   integer incrementAndGet                        if  i   2    1                            throw new NumberFormatException  Very odd exception                          else                           return i                                                            noOfRetry 10                   delayInterval 10  TimeUnit MILLISECONDS                   retryPredicate value - gt  value  lt   6                   retryOn NumberFormatException class  EOFException class                   build                    retry            Assert assertEquals 8  result intValue           catch  Throwable throwable            Assert fail

User · Answer

Here is my solution similar to some others can wrap a function  but allows you to get the functions return value  if it suceeds                  Wraps a function with retry logic allowing exceptions to be caught and retires made                 param function the function to retry         param maxRetries maximum number of retires before failing         param delay time to wait between each retry         param allowedExceptionTypes exception types where if caught a retry will be performed         param  lt V gt  return type of the function         return the value returned by the function if successful         throws Exception Either an unexpected exception from the function or a   link RuntimeException  if maxRetries is exceeded              SafeVarargs     public static  lt V gt  V runWithRetriesAndDelay Callable lt V gt  function  int maxRetries  Duration delay  Class lt   extends Exception gt     allowedExceptionTypes  throws Exception           final Set lt Class lt   extends Exception gt  gt  exceptions   new HashSet lt  gt  Arrays asList allowedExceptionTypes            for int i   1  i  lt   maxRetries  i                  try                   return function call                  catch  Exception e                    if exceptions contains e getClass                             An exception of an expected type                     System out println  Attempt      i         maxRetries      Caught exception      e getClass                                  Pause for the delay time                     Thread sleep delay toMillis                      else                          An unexpected exception type                     throw e                                                    throw new RuntimeException maxRetries     retries exceeded

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A simple way to solve the issue would be to wrap the try catch in a while loop and maintain a count  This way you could prevent an infinite loop by checking a count against some other variable while maintaining a log of your failures  It isn t the most exquisite solution  but it would work

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https   github com tusharmndr retry-function-wrapper tree master src main java io  int MAX RETRY   3   RetryUtil  lt Boolean gt retry MAX RETRY    - gt          Function to retry     return true

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The issue with the remaining solutions is that  the correspondent function tries continuously without a time interval in-between  thus over flooding the stack   Why not just trying only every second and ad eternum   Here a solution using setTimeout and a recursive function     x000D   x000D   function    x000D    try  x000D      Run      tries for the 1st time  but Run   as function is not yet defined x000D      x000D    catch e   x000D       function retry    x000D        setTimeout function    x000D          try  x000D            console log  trying       x000D            Run    x000D            console log  success     x000D            x000D          catch e   x000D            retry      calls recursively x000D            x000D           1000     tries every second x000D            x000D      x000D        x000D   x000D   x000D   x000D    after 5 seconds  defines Run as a global function x000D  var Run  x000D  setTimeout function    x000D    Run   function      x000D     5000   x000D   x000D   x000D    Replace the function Run   by the function or code that you d like to retry every second

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In case it s useful  a couple more options to consider  all thrown together  stopfile instead of retries  sleep  continue larger loop  all possibly helpful    bigLoop   while  stopFileExists          try            do work       break            catch  ExpectedExceptionType e               could sleep in here  too             another option would be to  restart  some bigger loop  like        continue bigLoop                   more work

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Your exact scenario handled via Failsafe   RetryPolicy retryPolicy   new RetryPolicy      retryOn NearlyUnexpectedException class    Failsafe with retryPolicy     onRetry  r  f  - gt  fix the problem       run    - gt  some instruction       Pretty simple

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Use a do-while to design re-try block   boolean successful   false  int maxTries   3  do    try       something        success   true      catch Me ifUCan        maxTries--        while   successful    maxTries  gt  0

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Spring AOP and annotation based solution   Usage   RetryOperation is our custom annotation for the job     RetryOperation retryCount   1  waitSeconds   10  boolean someMethod   throws Exception       We ll need two things to accomplish this  1  an annotation interface  and 2  a spring aspect  Here s one way to implement these   The Annotation Interface   import java lang annotation      Target ElementType METHOD   Retention RetentionPolicy RUNTIME  public  interface RetryOperation       int retryCount        int waitSeconds        The Spring Aspect   import org aspectj lang ProceedingJoinPoint  import org aspectj lang annotation Around  import org aspectj lang annotation Aspect  import org aspectj lang reflect MethodSignature  import org slf4j Logger  import org slf4j LoggerFactory  import org springframework stereotype Component  import java lang reflect Method    Aspect  Component  public class RetryAspect        private static final Logger LOGGER   LoggerFactory getLogger RetryAspect class         Around value     annotation RetryOperation        public Object retryOperation ProceedingJoinPoint joinPoint  throws Throwable            Object response   null          Method method     MethodSignature  joinPoint getSignature    getMethod            RetryOperation annotation   method getAnnotation RetryOperation class           int retryCount   annotation retryCount            int waitSeconds   annotation waitSeconds            boolean successful   false           do               try                   response   joinPoint proceed                    successful   true                catch  Exception ex                    LOGGER info  Operation failed  retries remaining       retryCount                   retryCount--                  if  retryCount  lt  0                        throw ex                                    if  waitSeconds  gt  0                        LOGGER info  Waiting for    second s  before next retry   waitSeconds                       Thread sleep waitSeconds   1000l                                             while   successful            return response

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As usual  the best design depends on the particular circumstances  Usually though  I write something like   for  int retries   0   retries          try           return doSomething          catch  SomeException e            if  retries  lt  6                continue            else               throw e

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I know there are already many similar answers here  and mine is not much different  but I will post it anyway because it deals with a specific case issue   When dealing with the facebook Graph API in PHP you sometimes get an error  but immediately re-trying the same thing will give a positive result  for various magical Internet reasons that are beyond the scope of this question   In this case there is no need to fix any error  but to simply try again because there was some kind of  facebook error    This code is used immediately after creating a facebook session     try more than once because sometimes  facebook error   attempt   3  while  attempt--  gt  0           To validate the session      try                 facebook session- gt validate             attempt   0             catch  Facebook FacebookRequestException  ex                   Session not valid  Graph API returned an exception with the reason          if  attempt  lt   0   echo  ex- gt getMessage                 catch   Exception  ex                    Graph API returned info  but it may mismatch the current app or have expired          if  attempt  lt   0   echo  ex- gt getMessage                Also  by having the for loop count down to zero   attempt--  it makes it pretty easy to change the number of attempts in the future

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Obligatory  enterprisy  solution   public abstract class Operation       abstract public void doIt        public void handleException Exception cause              default impl  do nothing  log the exception  etc           public class OperationHelper       public static void doWithRetry int maxAttempts  Operation operation            for  int count   0  count  lt  maxAttempts  count                  try                   operation doIt                    count   maxAttempts    don t retry               catch  Exception e                    operation handleException e                                     And to call   OperationHelper doWithRetry 5  new Operation          Override public void doIt               do some stuff            Override public void handleException Exception cause              recover from the Exception

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Here a reusable and more generic approach for Java 8  that does not require external libraries   public interface IUnreliable lt T extends Exception gt        void tryRun     throws T     public static  lt T extends Exception gt  void retry  int retryCount  IUnreliable lt T gt  runnable  throws T       for  int retries   0   retries              try               runnable tryRun                return            catch  Exception e                if  retries  lt  retryCount                    continue                else                   throw e                                    Usage    Test public void demo   throws IOException       retry 3     - gt            new File   tmp test txt   createNewFile

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Im not sure if this is the  Professional  way to do it and i m not entirely sure if it works for everything   boolean gotError   false   do       try              Code You re Trying       catch   FileNotFoundException ex                Exception         gotError   true          while   gotError   true

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simple int MAX   3   int count   0  while  true        try                        break        catch  Exception e            if  count    lt  MAX                continue                                          break

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Although try catch into while is well-known and good strategy I want to suggest you recursive call   void retry int i  int limit        try          catch  SomeException e               handle exception         if  i  gt   limit                throw e      variant  wrap the exception  e g  throw new RuntimeException e                     retry i    limit

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Below snippet execute some code snippet  If you got any error while executing the code snippet  sleep for M milliseconds and retry  Reference link   public void retryAndExecuteErrorProneCode int noOfTimesToRetry  CodeSnippet codeSnippet  int sleepTimeInMillis    throws InterruptedException     int currentExecutionCount   0   boolean codeExecuted   false    while  currentExecutionCount  lt  noOfTimesToRetry      try      codeSnippet errorProneCode       System out println  Code executed successfully           codeExecuted   true     break      catch  Exception e          Retry after 100 milliseconds    TimeUnit MILLISECONDS sleep sleepTimeInMillis      System out println e getMessage         finally      currentExecutionCount             if   codeExecuted    throw new RuntimeException  Can t execute the code within given retries       noOfTimesToRetry

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All a Try-Catch does is allow your program to fail gracefully   In a catch statement  you generally try to log the error  and maybe roll back changes if you need to     bool finished   false   while finished    false        try                 your code here         finished   true           catch exception ex                log error  there was an error  ex

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You can use AOP and Java annotations from jcabi-aspects  I m a developer     RetryOnFailure attempts   3  delay   5  public String load URL url      return url openConnection   getContent        You could also use  Loggable and  LogException annotations

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Give it a try using springs  Retryable annotation   the below method will retry for 3 attempts when RuntimeException occurs   Retryable maxAttempts 3 value   RuntimeException class  backoff    Backoff delay   500   public void checkSpringRetry String str        if StringUtils equalsIgnoreCase str   R              LOGGER info  Inside retry                   throw new RuntimeException

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Simplifying  ach s previous solution into one file and using functional interfaces  public class OperationHelper        public static void doWithRetry int maxAttempts  Runnable operation  Consumer lt Exception gt  handle            for  int count   0  count  lt  maxAttempts  count                  try                   operation run                    count   maxAttempts    don t retry               catch  Exception e                    handle accept e

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Use a while loop with local status flag  Initialize the flag as false and set it to true when operation is successful e g  below     boolean success    false    while  success        try            some instruction              success   true         catch  NearlyUnexpectedException e          fix the problem                 This will keep retrying until its successful   If you want to retry only certain number of times then use a counter as well     boolean success    false    int count   0  MAX TRIES   10    while  success  amp  amp  count    lt  MAX TRIES        try            some instruction              success   true         catch  NearlyUnexpectedException e          fix the problem                 if  success         It wasn t successful after 10 retries       This will try max 10 times if not successful until then an will exit if its successful before hand

[java] How do you implement a re-try-catch?

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