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CompletableFuture.supplyAsync() without Lambda

I'm struggling with the functional style of Supplier<U>, etc and creating testable code.
So I have an InputStream that is split into chunks which are processed asynchronously, and I want to know when they are all done. To write testable code, I outsource the processing logic to its own Runnable:
public class StreamProcessor {
public CompletableFuture<Void> process(InputStream in) {
List<CompletableFuture> futures = new ArrayList<>();
while (true) {
try (SizeLimitInputStream chunkStream = new SizeLimitInputStream(in, 100)) {
byte[] data = IOUtils.toByteArray(chunkStream);
CompletableFuture<Void> f = CompletableFuture.runAsync(createTask(data));
futures.add(f);
} catch (EOFException ex) {
// end of stream reached
break;
} catch (IOException ex) {
return CompletableFuture.failedFuture(ex);
}
}
return CompletableFuture.allOf(futures.toArray(CompletableFuture<?>[]::new));
}
ChunkTask createTask(byte[] data) {
return new ChunkTask(data);
}
public class ChunkTask implements Runnable {
final byte[] data;
ChunkTask(byte[] data) {
this.data = data;
}
#Override
public void run() {
try {
// do something
} catch (Exception ex) {
// checked exceptions must be wrapped
throw new RuntimeException(ex);
}
}
}
}
This works well, but poses two problems:
The processing code cannot return anything; it's a Runnable after all.
Any checked exceptions caught inside ChunkTask.run() must be wrapped into a RuntimeException. Unwrapping the failed combined CompletableFuture returns the RuntimeException which needs to be unwrapped again to reach the original cause - in contrast to the IOException.
So I'm looking for a way to do this with CompletableFuture.supplyAsync(), but I can't figure out how to do this without lambdas (bad to test) or to return a CompletableFuture.failedFuture() from the processing logic.

I can think of two approaches:
1. With supplyAsync:
When using CompletableFuture.supplyAsync, you need a supplier instead of a runnable:
public static class ChunkTask implements Supplier<Object> {
final byte[] data;
ChunkTask(byte[] data) {
this.data = data;
}
#Override
public Object get() {
Object result = ...;
// Do something or throw an exception
return result;
}
}
and then:
CompletableFuture
.supplyAsync( new ChunkTask( data ) )
.whenComplete( (result, throwable) -> ... );
If an exception happens in Supplier.get(), it will b e propagated and you can see it in CompletableFuture.whenComplete, CompletableFuture.handle or CompletableFuture.exceptionally.
2. Passing a CompletableFuture to the thread
You can pass a CompletableFuture to ChunkTask:
public class ChunkTask implements Runnable {
final byte[] data;
private final CompletableFuture<Object> future;
ChunkTask(byte[] data, CompletableFuture<Object> future) {
this.data = data;
this.future = future;
}
#Override
public void run() {
try {
Object result = null;
// do something
future.complete( result );
} catch (Throwable ex) {
future.completeExceptionally( ex );
}
}
}
Then the logic becomes:
while (true) {
CompletableFuture<Object> f = new CompletableFuture<>();
try (SizeLimitInputStream chunkStream = new SizeLimitInputStream(in, 100)) {
byte[] data = IOUtils.toByteArray(chunkStream);
startThread(new ChunkTask(data, f));
futures.add(f);
} catch (EOFException ex) {
// end of stream reached
break;
} catch (IOException ex) {
f.completeExceptionally( ex );
return f;
}
}
Probably, Number 2 is the one that gives you more flexibility on how to manage the exception.

How to catch RuntimeExceptions from Executors without blocking?

I have an executor service that accepts new tasks :
ExecutorService executor = Executors.newFixedThreadPool(1000);
//stupid example with several parralel tasks
for (int i = 0; i < 1000; i++) {
try{
Runnable task = new Runnable() {
public void run() {
throw new RuntimeException("foo");
}
};
executor.submit(task);
}
catch (ExecutionException e) {
System.out.println(e.getMessage());
}
}
My problem is that I'm not able to catch any exception thrown by the Runnable, unless I'm doing this :
Future<?> future = executor.submit(task);
try {
future.get();
} catch (Exception e) {
System.out.println("############### exception :" + e.getMessage());
}
The problem is that future.get() is blocking, so if I 'm not able to run my tasks asynchronously and my tasks will not run in parallel, but sequentially.
I would like to be able to use Java 8 and CompletableFuture but I can't ...
Do you have any other idea?
Thanks

The code inside the Runnable is executing on a separate thread, so you must handle its exceptions inside the run() method.
If you need to gather all the exceptions for later handling, I would do something like this:
ExecutorService executor = Executors.newFixedThreadPool(1000);
final List<Exception> exceptions = // a place to put exceptions
Collections.synchronizedList(new ArrayList<Exception>());
for (int i = 0; i < 1000; i++) {
Runnable task = new Runnable() {
public void run() {
try {
throw new RuntimeException("foo");
} catch (Exception e) {
exceptions.add(e); // save the exception for later
}
}
};
executor.submit(task);
}
// wait for all the tasks to finish, then...
for (Exception e: exceptions) {
// whatever you want to do
}
Otherwise, if you just want to get information about each exception as it occurs:
Runnable task = new Runnable() {
public void run() {
try {
throw new RuntimeException("foo");
} catch (Exception e) {
e.printStackTrace();
}
}
};

Anything you need to do after the task asynchronously can be added to the task itself.
for (int i = 0; i < 1000; i++) {
final Runnable task = new Runnable() {
public void run() {
throw new RuntimeException("foo");
}
};
executor.submit(new Runnable() {
public void run() {
try {
task.run();
} catch (Throwable e) {
e.printStackTrace();
}
}
});
}
or you combine them into one Runnable.

This may not be the best solution but we could make a parent Runnable which will do the work of the actual Runnable. The parent will catch all the exceptions you need to know about. Here is slight convoluted approach:
public static void main(String[] args){
ExecutorService executor = Executors.newFixedThreadPool(1000);
//stupid example with several parralel tasks
for (int i = 0; i < 5; i++) {
Runnable task = new Runnable() {
public void run() {
throw new RuntimeException("foo");
}
};
ParentRunnable t = new ParentRunnable();
t.setRunnable(task, i);
executor.submit(t);
}
}
static class ParentRunnable implements Runnable {
Runnable r;
int index;
public void setRunnable(Runnable r, int index){
this.r = r;
this.index = index;
}
public void run() {
try{
System.out.println("\n" + index + "\n");
r.run();
}catch(Exception e){
e.printStackTrace();
}
}
}

stop Callable task in ExecutorService

I'm learning concurrency and made some naive program to play with ExecutorService and Future tasks.
Also I want to check why instanceof is bad in some cases.
public class Test {
static enum Some {
FOO;
}
static abstract class Foo {
public abstract Some getType();
}
static class FooExt extends Foo {
public Some getType() {
return Some.FOO;
}
}
public static void main(String[] args) {
ExecutorService service = Executors.newFixedThreadPool(2);
final CountDownLatch start = new CountDownLatch(1);
Future<Integer> f1 = service.submit(new Callable<Integer>() {
#Override
public Integer call() {
try {
start.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Task started...");
int a = 0;
Foo foo = new FooExt();
while (!Thread.currentThread().isInterrupted()) {
if (foo instanceof FooExt) {
a++;
}
}
System.out.println("Task ended...");
return a;
}
});
Future<Integer> f2 = service.submit(new Callable<Integer>() {
#Override
public Integer call() {
try {
start.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Task started...");
int a = 0;
Foo foo = new FooExt();
while (!Thread.currentThread().isInterrupted()) {
if (foo.getType() == Some.FOO) {
a++;
}
}
System.out.println("Task ended...");
return a;
}
});
start.countDown();
try {
Thread.sleep(100);
} catch (InterruptedException e) {
}
service.shutdownNow();
System.out.println("service is shutdowned...");
try {
System.out.println("instanceof: "+f1.get());
System.out.println("enum: "+f2.get());
} catch (Exception e) {
e.printStackTrace();
}
}
}
but unfortunately my code is never terminated, and I cant get any values from my tasks :(

Hi I have executed your program. I got the following output:
Task started...
Task started...
Task ended...
service is shutdowned...
Task ended...
instanceof: 1287184
enum: 1247375
This code terminates.

How do you implement a re-try-catch?

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.

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.

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 < 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
}
});

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 < 6) {
continue;
} else {
throw e;
}
}
}

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.

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 >= limit) {
throw e; // variant: wrap the exception, e.g. throw new RuntimeException(e);
}
retry(i++, limit);
}
}

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 < 0) {
throw ex;
}
if (waitSeconds > 0) {
LOGGER.info("Waiting for {} second(s) before next retry", waitSeconds);
Thread.sleep(waitSeconds * 1000l);
}
}
} while (!successful);
return response;
}
}

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 < config.MAX_SOME_OPERATION_RETRIES; tryCount++)
{
try {
completed = some_operation();
break;
}
catch (UnlikelyException e) {
lastException = e;
fix_the_problem();
}
}
if (!completed) {
reportError(lastException);
}

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 && count++ < 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.

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<T> {
T evaluate() throws Throwable;
}
public interface RetryPredicate<T> {
boolean shouldRetry(T t);
}
public class RetryOperation<T> {
private RetryConsumer<T> retryConsumer;
private int noOfRetry;
private int delayInterval;
private TimeUnit timeUnit;
private RetryPredicate<T> retryPredicate;
private List<Class<? extends Throwable>> exceptionList;
public static class OperationBuilder<T> {
private RetryConsumer<T> iRetryConsumer;
private int iNoOfRetry;
private int iDelayInterval;
private TimeUnit iTimeUnit;
private RetryPredicate<T> iRetryPredicate;
private Class<? extends Throwable>[] exceptionClasses;
private OperationBuilder() {
}
public OperationBuilder<T> retryConsumer(final RetryConsumer<T> retryConsumer) {
this.iRetryConsumer = retryConsumer;
return this;
}
public OperationBuilder<T> noOfRetry(final int noOfRetry) {
this.iNoOfRetry = noOfRetry;
return this;
}
public OperationBuilder<T> delayInterval(final int delayInterval, final TimeUnit timeUnit) {
this.iDelayInterval = delayInterval;
this.iTimeUnit = timeUnit;
return this;
}
public OperationBuilder<T> retryPredicate(final RetryPredicate<T> retryPredicate) {
this.iRetryPredicate = retryPredicate;
return this;
}
#SafeVarargs
public final OperationBuilder<T> retryOn(final Class<? extends Throwable>... exceptionClasses) {
this.exceptionClasses = exceptionClasses;
return this;
}
public RetryOperation<T> build() {
if (Objects.isNull(iRetryConsumer)) {
throw new RuntimeException("'#retryConsumer:RetryConsumer<T>' not set");
}
List<Class<? extends Throwable>> exceptionList = new ArrayList<>();
if (Objects.nonNull(exceptionClasses) && exceptionClasses.length > 0) {
exceptionList = Arrays.asList(exceptionClasses);
}
iNoOfRetry = iNoOfRetry == 0 ? 1 : 0;
iTimeUnit = Objects.isNull(iTimeUnit) ? TimeUnit.MILLISECONDS : iTimeUnit;
return new RetryOperation<>(iRetryConsumer, iNoOfRetry, iDelayInterval, iTimeUnit, iRetryPredicate, exceptionList);
}
}
public static <T> OperationBuilder<T> newBuilder() {
return new OperationBuilder<>();
}
private RetryOperation(RetryConsumer<T> retryConsumer, int noOfRetry, int delayInterval, TimeUnit timeUnit,
RetryPredicate<T> retryPredicate, List<Class<? extends Throwable>> 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 < 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 < noOfRetry && delayInterval > 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.<Integer>newBuilder()
.retryConsumer(() -> {
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 -> value <= 6)
.retryOn(NumberFormatException.class, EOFException.class)
.build()
.retry();
Assert.assertEquals(8, result.intValue());
} catch (Throwable throwable) {
Assert.fail();
}
}

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.

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
}

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<Exception> 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 && maxRetries > 0);
if (err != null) {
throw err;
}
}
Usage:
runWithRetry(() -> {
// do something
}, TimeoutException.class, 5)

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");
}
}

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 > 0)

Here a reusable and more generic approach for Java 8+ that does not require external libraries:
public interface IUnreliable<T extends Exception>
{
void tryRun ( ) throws T;
}
public static <T extends Exception> void retry (int retryCount, IUnreliable<T> runnable) throws T {
for (int retries = 0;; retries++) {
try {
runnable.tryRun();
return;
} catch (Exception e) {
if (retries < retryCount) {
continue;
} else {
throw e;
}
}
}
}
Usage:
#Test
public void demo() throws IOException {
retry(3, () -> {
new File("/tmp/test.txt").createNewFile();
});
}

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(() -> System.out.println("Success, There is no result because this is a runnable."))
.onRetry((retryCount, e) -> System.out.println("Retry count: " + retryCount + ", Exception message: " + e.getMessage()))
.onFailure(e -> System.out.println("Failure: Exception message: " + e.getMessage()))
.run(new ExampleRunnable());
Instead of new ExampleRunnable() you can pass your own anonymous function.

Simplifying #ach's previous solution into one file and using functional interfaces.
public class OperationHelper {
public static void doWithRetry(int maxAttempts, Runnable operation, Consumer<Exception> handle) {
for (int count = 0; count < maxAttempts; count++) {
try {
operation.run();
count = maxAttempts; //don't retry
} catch (Exception e) {
handle.accept(e);
}
}
}
}

simple
int MAX = 3;
int count = 0;
while (true) {
try {
...
break;
} catch (Exception e) {
if (count++ < MAX) {
continue;
}
...
break;
}
}

https://onlinegdb.com/a-7RsL1Gh
public void doSomething() throws Exception{
final int MAX_TRIES = 10;
int count = 0;
while(count++ < MAX_TRIES){
try{
System.out.println("trying");
causeIssue(count); // throws error/exception till count 2
System.out.println("trying successful");
break; // break on success
} catch (Exception e){
System.out.println("caught, logging Exception:" + count);
} catch (Error e){
System.out.println("caught, logging Error:" + count);
}
}
}
Output:
trying
caught, logging Error:1
trying
caught, logging Error:2
trying
trying successful

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-- > 0)
{
// To validate the session:
try
{
$facebook_session->validate();
$attempt = 0;
}
catch (Facebook\FacebookRequestException $ex)
{
// Session not valid, Graph API returned an exception with the reason.
if($attempt <= 0){ echo $ex->getMessage(); }
}
catch (\Exception $ex)
{
// Graph API returned info, but it may mismatch the current app or have expired.
if($attempt <= 0){ echo $ex->getMessage(); }
}
}
Also, by having the for loop count down to zero ($attempt--) it makes it pretty easy to change the number of attempts in the future.

following is my solution with very simple approach!
while (true) {
try {
/// Statement what may cause an error;
break;
} catch (Exception e) {
}
}

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 );

https://github.com/tusharmndr/retry-function-wrapper/tree/master/src/main/java/io
int MAX_RETRY = 3;
RetryUtil.<Boolean>retry(MAX_RETRY,() -> {
//Function to retry
return true;
});

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:
(function(){
try{
Run(); //tries for the 1st time, but Run() as function is not yet defined
}
catch(e){
(function retry(){
setTimeout(function(){
try{
console.log("trying...");
Run();
console.log("success!");
}
catch(e){
retry(); //calls recursively
}
}, 1000); //tries every second
}());
}
})();
//after 5 seconds, defines Run as a global function
var Run;
setTimeout(function(){
Run = function(){};
}, 5000);
Replace the function Run() by the function or code that you'd like to retry every second.

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();
}
}

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 < 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);
}

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 <V> 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 <V> V runWithRetriesAndDelay(Callable<V> function, int maxRetries, Duration delay, Class<? extends Exception>... allowedExceptionTypes) throws Exception {
final Set<Class<? extends Exception>> exceptions = new HashSet<>(Arrays.asList(allowedExceptionTypes));
for(int i = 1; i <= 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");
}

This Solution allows you to configure a reusable functionality for retrying based on certain exception without using any external libraries
// Create a Function that suits your need .
#FunctionalInterface
public interface ThrowableBiFunction<U,T,R> {
R apply(U u ,T t) throws Exception;
}
//Here's the crux of the solution
public interface ExceptionRetryable<T, U, R> {
int getRetries();
List<Class<? extends Exception>> getRetryableExceptions();
default R execute(ThrowableBiFunction<T, U, R> function, T t, U u) throws Exception {
int numberOfRetries = getRetries();
return execute(function, t, u, numberOfRetries);
}
default R execute(ThrowableBiFunction<T, U, R> function, T t, U u, int retryCount) throws Exception {
try {
log.info(" Attempting to execute ExceptionRetryable#execute ,Number of remaining retries {} ",retryCount);
return function.apply(t, u);
} catch (Exception e) {
log.info(" error occurred in ExceptionRetryable#execute",e);
if (retryCount == 0)
throw e;
for (Class exp : getRetryableExceptions()) {
if (e.getClass() == exp) {
return execute(function, t, u, retryCount - 1);
}
}
throw e;
}
}
}
// create an implementation for exception retryable
public class TestRetryable implements ExceptionRetryable<String, String, List<String>> {
#Override
public int getRetries() {
return 10;
}
#Override
public List<Class<? extends Exception>> getRetryableExceptions() {
return Arrays.asList(new Exception1().getClass(), new Exception2().getClass());
;
}
}
// Finally create a ThrowableBiFunction that encapsulates that piece of code that needs to be retried on exception and an instance of ExceptionRetryable
TestRetryable retryable = new TestRetryable();
ThrowableBiFunction<Integer,Long, String> testRetrablefcn = { i, l ->
// your code goes here
};
Integer i = 0;
Long l = 1l;
String output = testRetrablefcn.execute(testRetrablefcn,i,l);

Production ready code:
#FunctionalInterface
public interface Operation {
void doCall() throws IOException;
default void handleException(Exception e) {
//Your custom default implementation
}
public class OperationHelper {
public static void doWithRetry(int maxAttempts, Operation operation) {
for (int count = 0; count <= maxAttempts; count++) {
try {
operation.doCall();
return;
} catch (Exception e) {
if (count == maxAttempts) {
e.printStackTrace();
return;
} else {
operation.handleException(e);
}
}
}
}
}
Usage with default implementation in code:
OperationHelper.doWithRetry(10,
() -> //do your job );
Usage when custom exception handle is needed:
OperationHelper.doWithRetry(10, new Operation() {
#Override public void doIt() {
//do some stuff
}
#Override public void handleException(Exception cause) {
//recover from the Exception
}
});

Invoke more than one webservice using separate thread in java?

I have to call more than one webservice in one method each webservice is executed by separate thread in concurrent/parellel. Every web service will return one ArrayList. Note: may chance some webservices will fail or take more time process response in this case i have to skip these failure result. How can I achieve this? I tried this sample code.
public class MultiThreadsEx{
public class Task implements Runnable {
private Object result;
private String id;
int maxRowCount = 0;
public Task(String id) {
this.id = id;
}
public Object getResult() {
return result;
}
public void run() {
try {
System.out.println("Running id=" + id+" at "+Utilities.getCurrentJavaDate("DD/MM/YYYY HH:MM:SS"));
if(id.equalsIgnoreCase("1")){
/**Getting Details from Amazon WS*/
maxRowCount = AmazonUtils.getweather(cityname);
}else if(id.equalsIgnoreCase("2")){
/**Getting Details from Google WS* /
maxRowCount = GoogleUtils.getWeather(cityName);
}
// call web service
//Thread.sleep(1000);
//result = id + " more";
result = maxRowCount;
} catch (InterruptedException e) {
// TODO do something with the error
throw new RuntimeException("caught InterruptedException", e);
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
public static void runInParallel(Runnable runnable1, Runnable runnable2) {
try {
Thread t1 = new Thread(runnable1);
Thread t2 = new Thread(runnable2);
t1.start();
t2.start();
} catch (Exception e) {
// TODO do something nice with exception
throw new RuntimeException("caught InterruptedException", e);
}
}
public void foo() {
try {
Task task1 = new Task("1");
Task task2 = new Task("2");
runInParallel(task1, task2);
System.out.println("task1 = " + task1.getResult()+" at "+Utilities.getCurrentJavaDate("DD/MM/YYYY HH:MM:SS"));
System.out.println("task2 = " + task2.getResult()+" at "+Utilities.getCurrentJavaDate("DD/MM/YYYY HH:MM:SS"));
} catch (Exception e) {
//TODO Auto-generated catch block
e.printStackTrace();
}
}
But run() return type is void so how can return result? Examples are highly appreciated. I am new to multithread/concurrent threads concept so if I'm doing anything wrong, please point me in the right direction.

Consider replacing Runnable - run with Callable - call. This will allow you to return a result from your thread task:
public class Task implements Callable<Object> {
private Object result;
public Object call() {
// compute result
return result;
}
}
Now use an ExecutorService:
public static void runInParallel(Callable<Object> c1, Callable<Object> c2) {
ExecutorService exec = Executors.newFixedThreadPool(2);
Future<Object> f1 = exec.submit(c1);
Future<Object> f2 = exec.submit(c2);
}
Later in the code you can use f1.get() and f2.get() to wait for the results of the tasks.

The usual way to communicate the results of a Runnable back to the object which created it is by passing the creating object to the constructor of the Runnable. When the task is finished, you can call a method in the creating object and pass the result data.