How to use CompletableFuture's whenComplete and exceptionally functions for exception handling in Java

In Java's asynchronous programming, CompletableFuture is a very useful class. It can help us perform callbacks after the asynchronous operation is completed, and has a good exception handling mechanism. This article will introduce how to use CompletableFuture's whenComplete and exceptionally functions for exception handling.

1. The whenComplete function of CompletableFuture

The whenComplete function of CompletableFuture is a callback function, which will be called when the asynchronous operation is completed. When the asynchronous operation completes normally, when the first parameter of Complete is the result of the asynchronous operation, we can operate on the result in the callback function. When an asynchronous operation fails, it passes the exception to the callback function as the second parameter of whenComplete.

Sample code:

CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> {
    return "hello world";
});
future.whenComplete((result, throwable) -> {
    if (throwable == null) {
        System.out.println("result: " + result);
    } else {
        System.out.println("exception: " + throwable.getMessage());
    }
});

In the above code, we first created a CompletableFuture instance and used the supplyAsync method to asynchronously execute a task. This task returned a string "hello world ". We use the whenComplete function as a callback function to process the asynchronous operation after it is completed. If the asynchronous operation completes normally, we print out the result; if the asynchronous operation fails, we print out the exception information.

2. The exceptionally function of CompletableFuture

The exceptionally function of CompletableFuture is also a callback function, but it will only be called when an exception occurs in the asynchronous operation. Its return value is a new CompletableFuture instance, used to handle exception situations. We can handle exceptions in the exceptionally function and return a default value or other value.

Sample code:

CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> {
    throw new RuntimeException("exception occurred");
});
CompletableFuture<String> result = future.exceptionally(throwable -> {
    return "default value";
});
result.whenComplete((value, throwable) -> {
    if (throwable == null) {
        System.out.println("result: " + value);
    } else {
        System.out.println("exception: " + throwable.getMessage());
    }
});

In the above code, we use the supplyAsync method to perform a task that will throw an exception. We use the exceptionally function to handle exceptions, and its return value is a new CompletableFuture instance. In the callback function, we can process the results. If the exception is not caught, we will print out the exception information; if the exception is caught and handled, we will print out the returned default value.

Summary

This article introduces how to use CompletableFuture's whenComplete and exceptionally functions for exception handling. Using CompletableFuture's asynchronous callback function and exception handling mechanism can greatly improve the efficiency and reliability of Java's asynchronous programming.

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