fm.last.commons.lang.templates

Class RetryTemplate

java.lang.Object

fm.last.commons.lang.templates.RetryTemplate

public class RetryTemplate
extends Object

Simple(ish) class for retrying something on exceptions. Useful for doing things which may timeout or stop responding temporarily. It logs a warning on each retry (with log4j). If it exceeds the maximum attempts the last exception is thrown. It catches all exceptions, and does not catch an Error. Here is an example of usage, it tries 10 times with a 5 second pause between each try.

 try {
   new RetryTemplate(10, 5).execute(new RetryTemplate.RetryCallback() {
     public Object doWithRetry() throws Exception {
       // dosome processing
       return null;
     }
   });
 } catch (Exception e) {
   log.fatal(e);
   throw e;
 }
 

If you wish to return a value, you can return it as an object and cast it.

 String hello = (String) new RetryTemplate(10, 5).execute(new RetryTemplate.RetryCallback() {
   public Object doWithRetry() throws Exception {
     return "hello world";
   }
 });
 System.out.println(hello);
 

To use variables within the function either declare it final, or extend RetryCallback. Eg,

 final String hello = "hello world";
 new RetryTemplate(10, 5).execute(new RetryTemplate.RetryCallback() {
   public Object doWithRetry() throws Exception {
     System.out.println(hello);
     return null;
   }
 });
 

or

 public class PrintCallback extends RetryTemplate.RetryCallback {
   private String str = null;
 
   public PrintCallback(String str) {
     this.str = str;
   }
 
   public Object doWithRetry() throws Exception {
     System.out.println(str);
     return null;
   }
 }
 new RetryTemplate(10, 5).execute(new PrintCallback("hello world"));
 

You can optionally override the RetryCallback onException function if for example you wish to log each exception or close and reopen a connection. Eg:

 SomeConnectionClass connection = bla bla;
 
 new RetryTemplate(10,5).execute(new RetryTemplate.RetryCallback(){
   public Object doWithRetry() throws Exception {
     // dosome processing
     return null;
   }
   
   public void onException(Exception e) {
     log.warn(e);
     connection.close();
     connection.open();
   }
 });
 

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static class	RetryTemplate.RetryCallback<T> Abstract base class for use with RetryTemplate

Constructor Summary

Constructors

Constructor and Description
RetryTemplate()
RetryTemplate(int maxAttempts, int sleepSecs)

Method Summary

Methods

Modifier and Type	Method and Description
<T> T	execute(RetryTemplate.RetryCallback<T> callback) Calls doWithRetry on the callback object while an exception occurs or we reach the maximum retries.
protected void	retrySleep()

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

RetryTemplate

public RetryTemplate()

RetryTemplate

public RetryTemplate(int maxAttempts,
             int sleepSecs)

Method Detail

execute

public <T> T execute(RetryTemplate.RetryCallback<T> callback)
          throws Exception

Calls doWithRetry on the callback object while an exception occurs or we reach the maximum retries.

Throws:

Exception

retrySleep

protected void retrySleep()