How to Supply Fallback Values

If an operation fails, it is often possible to replace the failure with an alternative result: a fallback.

`@Fallback`

Using a Fallback Method

The @Fallback annotation allows specifying a method that should be called in case an exception is thrown. The fallback method must have the same parameter types as the guarded method, and must return a type that is assignable to the guarded method’s return type.

@ApplicationScoped
public class MyService {
    @Fallback(fallbackMethod = "fallback") (1)
    public String hello() {
        ...
    }

    private String fallback() {
        return "fallback";
    }
}

1	Declares that if `hello()` throws an exception, the `fallback()` method is called and its result is returned instead.

In this particular case, if hello() throws an exception, that exception is swallowed and the string "fallback" is returned. It is possible to specify that fallback should only apply to certain exception types or that it should be skipped for certain exception types.

Using a Fallback Handler

Alternatively, the @Fallback annotation allows specifying a fallback handler that should be called.

@ApplicationScoped
public class MyService {
    @Fallback(MyFallback.class) (1)
    public String hello() {
        ...
    }
}

1	Declares that if `hello()` throws an exception, the `MyFallback` handler is used to obtain a fallback value.

public class MyFallback implements FallbackHandler<String> { (1)
    String handle(ExecutionContext context) { (2)
        return "fallback";
    }
}

1	A fallback handler must implement the `FallbackHandler` interface.
2	The `ExecutionContext` provides access to the guarded method, its parameter values, and the thrown exception.

Links

For more information, see the Fallback reference guide.