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README.md

ci.spring.io Javadocs

This project provides declarative retry support for Spring applications. It is used in Spring Batch, Spring Integration, and others. Imperative retry is also supported for explicit usage.

Quick Start

Declarative example

@Configuration
@EnableRetry
public class Application {

    @Bean
    public Service service() {
        return new Service();
    }

}

@Service
class Service {
    @Retryable(RemoteAccessException.class)
    public void service() {
        // ... do something
    }
    @Recover
    public void recover(RemoteAccessException e) {
       // ... panic
    }
}

Call the service method and, if it fails with a RemoteAccessException, it retries (up to three times by default), and then execute the "recover" method if unsuccessful. There are various options in the @Retryable annotation attributes for including and excluding exception types, limiting the number of retries and the policy for backoff.

The declarative approach to applying retry handling by using the @Retryable annotation shown earlier has an additional runtime dependency on AOP classes. For details on how to resolve this dependency in your project, see the 'Java Configuration for Retry Proxies' section below.

Imperative example

Since version 1.3:

RetryTemplate template = RetryTemplate.builder()
				.maxAttempts(3)
				.fixedBackoff(1000)
				.retryOn(RemoteAccessException.class)
				.build();

template.execute(ctx -> {
    // ... do something
});

Older versions: see example in RetryTemplate section.

Building

Spring Retry requires Java 1.7 and Maven 3.0.5 (or greater). To build, run the following Maven command:

$ mvn install

Features and API

This section discusses the features of Spring Retry and shows how to use its API.

Using RetryTemplate

To make processing more robust and less prone to failure, it sometimes helps to automatically retry a failed operation, in case it might succeed on a subsequent attempt. Errors that are susceptible to this kind of treatment are transient in nature. For example, a remote call to a web service or an RMI service that fails because of a network glitch or a DeadLockLoserException in a database update may resolve itself after a short wait. To automate the retry of such operations, Spring Retry has the RetryOperations strategy. The RetryOperations interface definition follows:

public interface RetryOperations {

    <T> T execute(RetryCallback<T> retryCallback) throws Exception;

    <T> T execute(RetryCallback<T> retryCallback, RecoveryCallback<T> recoveryCallback)
        throws Exception;

    <T> T execute(RetryCallback<T> retryCallback, RetryState retryState)
        throws Exception, ExhaustedRetryException;

    <T> T execute(RetryCallback<T> retryCallback, RecoveryCallback<T> recoveryCallback,
        RetryState retryState) throws Exception;

}

The basic callback is a simple interface that lets you insert some business logic to be retried:

public interface RetryCallback<T> {

    T doWithRetry(RetryContext context) throws Throwable;

}

The callback is executed and, if it fails (by throwing an Exception), it is retried until either it is successful or the implementation decides to abort. There are a number of overloaded execute methods in the RetryOperations interface, to deal with various use cases for recovery when all retry attempts are exhausted and with retry state, which lets clients and implementations store information between calls (more on this later).

The simplest general purpose implementation of RetryOperations is RetryTemplate. The following example shows how to use it:

RetryTemplate template = new RetryTemplate();

TimeoutRetryPolicy policy = new TimeoutRetryPolicy();
policy.setTimeout(30000L);

template.setRetryPolicy(policy);

Foo result = template.execute(new RetryCallback<Foo>() {

    public Foo doWithRetry(RetryContext context) {
        // Do stuff that might fail, e.g. webservice operation
        return result;
    }

});

In the preceding example, we execute a web service call and return the result to the user. If that call fails, it is retried until a timeout is reached.

Since version 1.3, fluent configuration of RetryTemplate is also available:

RetryTemplate.builder()
      .maxAttempts(10)
      .exponentialBackoff(100, 2, 10000)
      .retryOn(IOException.class)
      .traversingCauses()
      .build();
 
RetryTemplate.builder()
      .fixedBackoff(10)
      .withinMillis(3000)
      .build();
 
RetryTemplate.builder()
      .infiniteRetry()
      .retryOn(IOException.class)
      .uniformRandomBackoff(1000, 3000)
      .build();

Using RetryContext

The method parameter for the RetryCallback is a RetryContext. Many callbacks ignore the context. However, if necessary, you can use it as an attribute bag to store data for the duration of the iteration.

A RetryContext has a parent context if there is a nested retry in progress in the same thread. The parent context is occasionally useful for storing data that needs to be shared between calls to execute.

Using RecoveryCallback

When a retry is exhausted, the RetryOperations can pass control to a different callback: RecoveryCallback. To use this feature, clients can pass in the callbacks together to the same method, as the following example shows:

Foo foo = template.execute(new RetryCallback<Foo>() {
    public Foo doWithRetry(RetryContext context) {
        // business logic here
    },
  new RecoveryCallback<Foo>() {
    Foo recover(RetryContext context) throws Exception {
          // recover logic here
    }
});

If the business logic does not succeed before the template decides to abort, the client is given the chance to do some alternate processing through the recovery callback.

Stateless Retry

In the simplest case, a retry is just a while loop: the RetryTemplate can keep trying until it either succeeds or fails. The RetryContext contains some state to determine whether to retry or abort. However, this state is on the stack, and there is no need to store it anywhere globally. Consequently, we call this stateless retry. The distinction between stateless and stateful retry is contained in the implementation of RetryPolicy (RetryTemplate can handle both). In a stateless retry, the callback is always executed in the same thread as when it failed on retry.

Stateful Retry

Where the failure has caused a transactional resource to become invalid, there are some special considerations. This does not apply to a simple remote call, because there is no transactional resource (usually), but it does sometimes apply to a database update, especially when using Hibernate. In this case, it only makes sense to rethrow the exception that called the failure immediately so that the transaction can roll back and we can start a new (and valid) one.

In these cases, a stateless retry is not good enough because the re-throw and roll back necessarily involve leaving the RetryOperations.execute() method and potentially losing the context that was on the stack. To avoid losing the context, we have to introduce a storage strategy to lift it off the stack and put it (at a minimum) in heap storage. For this purpose, Spring Retry provides a storage strategy called RetryContextCache, which you can inject into the RetryTemplate. The default implementation of the RetryContextCache is in-memory, using a simple Map. It has a strictly enforced maximum capacity, to avoid memory leaks, but it does not have any advanced cache features (such as time to live). You should consider injecting a Map that has those features if you need them. For advanced usage with multiple processes in a clustered environment, you might also consider implementing the RetryContextCache with a cluster cache of some sort (though, even in a clustered environment, this might be overkill).

Part of the responsibility of the RetryOperations is to recognize the failed operations when they come back in a new execution (and usually wrapped in a new transaction). To facilitate this, Spring Retry provides the RetryState abstraction. This works in conjunction with special execute methods in the RetryOperations.

The failed operations are recognized by identifying the state across multiple invocations of the retry. To identify the state, you can provide a RetryState object that is responsible for returning a unique key that identifies the item. The identifier is used as a key in the RetryContextCache.

Warning: Be very careful with the implementation of Object.equals() and Object.hashCode() in the key returned by RetryState. The best advice is to use a business key to identify the items. In the case of a JMS message, you can use the message ID.

When the retry is exhausted, you also have the option to handle the failed item in a different way, instead of calling the RetryCallback (which is now presumed to be likely to fail). As in the stateless case, this option is provided by the RecoveryCallback, which you can provide by passing it in to the execute method of RetryOperations.

The decision to retry or not is actually delegated to a regular RetryPolicy, so the usual concerns about limits and timeouts can be injected there (see the next section).

Retry Policies

Inside a RetryTemplate, the decision to retry or fail in the execute method is determined by a RetryPolicy, which is also a factory for the RetryContext. The RetryTemplate is responsible for using the current policy to create a RetryContext and passing that in to the RetryCallback at every attempt. After a callback fails, the RetryTemplate has to make a call to the RetryPolicy to ask it to update its state (which is stored in RetryContext). It then asks the policy if another attempt can be made. If another attempt cannot be made (for example, because a limit has been reached or a timeout has been detected), the policy is also responsible for identifying the exhausted state -- but not for handling the exception. RetryTemplate throws the original exception, except in the stateful case, when no recovery is available. In that case, it throws RetryExhaustedException. You can also set a flag in the RetryTemplate to have it unconditionally throw the original exception from the callback (that is, from user code) instead.

Tip: Failures are inherently either retryable or not -- if the same exception is always going to be thrown from the business logic, it does not help to retry it. So you should not retry on all exception types. Rather, try to focus on only those exceptions that you expect to be retryable. It is not usually harmful to the business logic to retry more aggressively, but it is wasteful, because, if a failure is deterministic, time is spent retrying something that you know in advance is fatal.

Spring Retry provides some simple general-purpose implementations of stateless RetryPolicy (for example, a SimpleRetryPolicy), and the TimeoutRetryPolicy used in the preceding example.

The SimpleRetryPolicy allows a retry on any of a named list of exception types, up to a fixed number of times. The following example shows how to use it:

// Set the max attempts including the initial attempt before retrying
// and retry on all exceptions (this is the default):
SimpleRetryPolicy policy = new SimpleRetryPolicy(5, Collections.singletonMap(Exception.class, true));

// Use the policy...
RetryTemplate template = new RetryTemplate();
template.setRetryPolicy(policy);
template.execute(new RetryCallback<Foo>() {
    public Foo doWithRetry(RetryContext context) {
        // business logic here
    }
});

A more flexible implementation called ExceptionClassifierRetryPolicy is also available. It lets you configure different retry behavior for an arbitrary set of exception types through the ExceptionClassifier abstraction. The policy works by calling on the classifier to convert an exception into a delegate RetryPolicy, so, for example, one exception type can be retried more times before failure than another, by mapping it to a different policy.

You might need to implement your own retry policies for more customized decisions -- for instance, if there is a well-known, solution-specific, classification of exceptions into retryable and not retryable.

Backoff Policies

When retrying after a transient failure, it often helps to wait a bit before trying again, because (usually) the failure is caused by some problem that can be resolved only by waiting. If a RetryCallback fails, the RetryTemplate can pause execution according to the BackoffPolicy. The following listing shows the definition of the BackoffPolicy interface:

public interface BackoffPolicy {

    BackOffContext start(RetryContext context);

    void backOff(BackOffContext backOffContext)
        throws BackOffInterruptedException;

}

A BackoffPolicy is free to implement the backoff in any way it chooses. The policies provided by Spring Retry out of the box all use Object.wait(). A common use case is to backoff with an exponentially increasing wait period, to avoid two retries getting into lock step and both failing (a lesson learned from Ethernet). For this purpose, Spring Retry provides ExponentialBackoffPolicy. Spring Retry also provides randomized versions of delay policies that are quite useful to avoid resonating between related failures in a complex system.

Listeners

It is often useful to be able to receive additional callbacks for cross cutting concerns across a number of different retries. For this purpose, Spring Retry provides the RetryListener interface. The RetryTemplate lets you register RetryListener instances, and they are given callbacks with the RetryContext and Throwable (where available during the iteration).

The following listing shows the RetryListener interface:

public interface RetryListener {

    void open(RetryContext context, RetryCallback<T> callback);

    void onError(RetryContext context, RetryCallback<T> callback, Throwable e);

    void close(RetryContext context, RetryCallback<T> callback, Throwable e);
}

The open and close callbacks come before and after the entire retry in the simplest case, and onError applies to the individual RetryCallback calls. The close method might also receive a Throwable. If there has been an error, it is the last one thrown by the RetryCallback.

Note that when there is more than one listener, they are in a list, so there is an order. In this case, open is called in the same order, while onError and close are called in reverse order.

Listeners for reflective method invocations

When dealing with methods annotated with @Retryable or with Spring AOP intercepted methods, spring-retry provides the possibility to inspect in detail the method invocation within the RetryListener implementation.

Such a scenario could be particularly useful when there is a need to monitor how often a certain method call has been retried and expose it with detailed tagging information (e.g. : class name, method name, or even parameter values in some exotic cases).

template.registerListener(new MethodInvocationRetryListenerSupport() {
      @Override
      protected <T, E extends Throwable> void doClose(RetryContext context,
          MethodInvocationRetryCallback<T, E> callback, Throwable throwable) {
        monitoringTags.put(labelTagName, callback.getLabel());
        Method method = callback.getInvocation()
            .getMethod();
        monitoringTags.put(classTagName,
            method.getDeclaringClass().getSimpleName());
        monitoringTags.put(methodTagName, method.getName());

        // register a monitoring counter with appropriate tags
        // ...
      }
    });

Declarative Retry

Sometimes, you want to retry some business processing every time it happens. The classic example of this is the remote service call. Spring Retry provides an AOP interceptor that wraps a method call in a RetryOperations instance for exactly this purpose. The RetryOperationsInterceptor executes the intercepted method and retries on failure according to the RetryPolicy in the provided RepeatTemplate.

Java Configuration for Retry Proxies

You can add the @EnableRetry annotation to one of your @Configuration classes and use @Retryable on the methods (or on the type level for all methods) that you want to retry. You can also specify any number of retry listeners. The following example shows how to do so:

@Configuration
@EnableRetry
public class Application {

    @Bean
    public Service service() {
        return new Service();
    }

    @Bean public RetryListener retryListener1() {
        return new RetryListener() {...}
    }

    @Bean public RetryListener retryListener2() {
        return new RetryListener() {...}
    }

}

@Service
class Service {
    @Retryable(RemoteAccessException.class)
    public service() {
        // ... do something
    }
}

You can use the attributes of @Retryable to control the RetryPolicy and BackoffPolicy, as the following example shows:

@Service
class Service {
    @Retryable(maxAttempts=12, backoff=@Backoff(delay=100, maxDelay=500))
    public service() {
        // ... do something
    }
}

The preceding example creates a random backoff between 100 and 500 milliseconds and up to 12 attempts. There is also a stateful attribute (default: false) to control whether the retry is stateful or not. To use stateful retry, the intercepted method has to have arguments, since they are used to construct the cache key for the state.

The @EnableRetry annotation also looks for beans of type Sleeper and other strategies used in the RetryTemplate and interceptors to control the behavior of the retry at runtime.

The @EnableRetry annotation creates proxies for @Retryable beans, and the proxies (that is, the bean instances in the application) have the Retryable interface added to them. This is purely a marker interface, but it might be useful for other tools looking to apply retry advice (they should usually not bother if the bean already implements Retryable).

You can supply a recovery method, in case you want to take an alternative code path when the retry is exhausted. Methods should be declared in the same class as the @Retryable and marked @Recover. The return type must match the @Retryable method. The arguments for the recovery method can optionally include the exception that was thrown and (optionally) the arguments passed to the original retryable method (or a partial list of them as long as none are omitted). The following example shows how to do so:

@Service
class Service {
    @Retryable(RemoteAccessException.class)
    public void service(String str1, String str2) {
        // ... do something
    }
    @Recover
    public void recover(RemoteAccessException e, String str1, String str2) {
       // ... error handling making use of original args if required
    }
}

Version 1.2 introduces the ability to use expressions for certain properties. The following example show how to use expressions this way:

@Retryable(exceptionExpression="message.contains('this can be retried')")
public void service1() {
  ...
}

@Retryable(exceptionExpression="message.contains('this can be retried')")
public void service2() {
  ...
}

@Retryable(exceptionExpression="@exceptionChecker.shouldRetry(#root)",
    maxAttemptsExpression = "#{@integerFiveBean}",
  backoff = @Backoff(delayExpression = "#{1}", maxDelayExpression = "#{5}", multiplierExpression = "#{1.1}"))
public void service3() {
  ...
}

Since Spring Retry 1.2.5, for exceptionExpression, templated expressions (#{...}) are deprecated in favor of simple expression strings (message.contains('this can be retried')).

Expressions can contain property placeholders, such as #{${max.delay}} or #{@exceptionChecker.${retry.method}(#root)}. The following rules apply:

  • exceptionExpression is evaluated against the thrown exception as the #root object.
  • maxAttemptsExpression and the @BackOff expression attributes are evaluated once, during initialization. There is no root object for the evaluation but they can reference other beans in the context.

Additional Dependencies

The declarative approach to applying retry handling by using the @Retryable annotation shown earlier has an additional runtime dependency on AOP classes that need to be declared in your project. If your application is implemented by using Spring Boot, this dependency is best resolved by using the Spring Boot starter for AOP. For example, for Gradle, add the following line to your build.gradle:

    runtime('org.springframework.boot:spring-boot-starter-aop')

For non-Boot apps, you need to declare a runtime dependency on the latest version of AspectJ's aspectjweaver module. For example, for Gradle, you should add the following line to your build.gradle file:

    runtime('org.aspectj:aspectjweaver:1.8.13')

XML Configuration

The following example of declarative iteration uses Spring AOP to repeat a service call to a method called remoteCall:

<aop:config>
    <aop:pointcut id="transactional"
        expression="execution(* com..*Service.remoteCall(..))" />
    <aop:advisor pointcut-ref="transactional"
        advice-ref="retryAdvice" order="-1"/>
</aop:config>

<bean id="retryAdvice"
    class="org.springframework.retry.interceptor.RetryOperationsInterceptor"/>

For more detail on how to configure AOP interceptors, see the Spring User Guide.

The preceding example uses a default RetryTemplate inside the interceptor. To change the policies or listeners, you need only inject an instance of RetryTemplate into the interceptor.

Contributing

Spring Retry is released under the non-restrictive Apache 2.0 license and follows a very standard Github development process, using Github tracker for issues and merging pull requests into the master branch. If you want to contribute even something trivial, please do not hesitate, but do please follow the guidelines in the next paragraph.

Before we can accept a non-trivial patch or pull request, we need you to sign the contributor's agreement. Signing the contributor's agreement does not grant anyone commit rights to the main repository, but it does mean that we can accept your contributions, and you will get an author credit if we do. Active contributors might be asked to join the core team and be given the ability to merge pull requests.

Code of Conduct

This project adheres to the Contributor Covenant. By participating, you are expected to uphold this code. Please report unacceptable behavior to spring-code-of-conduct@pivotal.io.

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