The exception handling of CompleteFuture needs to be actively captured. The main methods include: 1. Use exceptionally() to provide a default value guarantee; 2. Use handle() to check results or exceptions at each step and handle them; 3. WhenComplete() is used to record logs or clean resources but not change the results. Unlike synchronous exceptions, exceptions in asynchronous tasks are encapsulated and are not thrown until the call to join() or get() is called. If not processed, it will cause silence failure. In addition, missing intermediate exception handling in chain calls and not traversing exceptions when combining multiple tasks will cause problems. It is recommended to use handle() to control the process in a unified manner and check the exception logic for the combined tasks one by one.
Handling Java's CompletableFuture exceptions is not as intuitive as traditional try-catch, but it is not too difficult. The key is to understand how exceptions propagate in asynchronous tasks and what methods do you have to catch and process them.
How do exceptions bubble in asynchronous tasks?
In the synchronization code, throwing an exception will directly interrupt the process and throw the call stack up layer by layer. But in CompletableFuture , if an exception is thrown at a certain stage, it will not "blast" immediately, but will encapsulate the exception and will not be actually thrown until someone calls get() or join() .
for example:
CompleteFuture<Integer> future = CompleteFuture.supplyAsync(() -> {
if (true) throw new RuntimeException("Oops");
return 100;
});
future.join(); // Exception will be triggered here So if you do not actively call get() , join() or do not set exception handling logic, the program may look like everything is normal, but in fact it is just "hiding" the error.
What are the most commonly used exception handling methods?
1. Use exceptionally() method to protect the situation
This method is a bit like the default alternative: as long as the previous task fails, go here.
CompleteFuture<Integer> future = CompleteFuture.supplyAsync(() -> {
throw new RuntimeException("Boom!");
}).exceptionally(ex -> {
System.out.println("Caught: " ex.getMessage());
return 42; // default value});This method is suitable for situations where you want to return a default result even if it fails.
2. Check every step using handle()
handle() is a more flexible way, regardless of whether the previous success or failure, it will be executed and a result or exception is passed.
CompleteFuture<Integer> future = CompleteFuture.supplyAsync(() -> {
return 10 / 0;
}).handle((result, ex) -> {
if (ex != null) {
System.out.println("Error occurred: " ex.getMessage());
return 0; // An error returns default value}
return result;
});This way you can process the results of each step, rather than just protecting the situation at the end.
3. Use whenComplete() to only record and not change the result
If you want to record logs or do some cleaning work, but don't want to affect the final result, you can use this method.
CompleteFuture<Integer> future = CompleteFuture.supplyAsync(() -> {
return 100;
}).whenComplete((result, ex) -> {
if (ex != null) {
System.out.println("Task failed with: " ex.getMessage());
} else {
System.out.println("Result is: " result);
}
}); Note: whenComplete() will not modify the result, nor can the exception be restored.
Common misunderstandings and precautions
Unprocessed exceptions result in silence failure
If you don't useget()orjoin(), and do not register an exception handler, exceptions in the task may never be discovered.Mistake of intermediate exception handling in chained calls
For example, if you usethenApply()in the middle and throw an exception, but there is noexceptionallyorhandle()in the subsequent process, the entire chain will fail.Be careful when combining multiple tasks
For example,allOf()andanyOf()behave differently.allOf()will complete if it encounters a failure, but you need to manually traverse each future to get the specific error.
Tips
- If you want to automatically try again after the task fails, you can resubmit the task in
exceptionally(). - For complex processes, it is recommended to use
handle()to control the behavior of each step. - If it is a log monitoring operation, use
whenComplete()first to avoid interfering with the main process. - When combining multiple asynchronous tasks, remember to check whether each subtask has exception handling logic.
Basically that's it. After all, the exception handling of CompletableFuture is to take the initiative to "catch" it, otherwise it is easy to miss the problem.
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