Spring AOP @Transactional

@Transactional

Propagation

Defines how transactions relate to each other. Common options

• Required: Code will always run in a transaction. Create a new transaction or reuse one if availble.
• Requires_new: Code will always run in a new transaction. Suspend current transaction if one exist.

Isolation

Defines the data contract between transactions.

• Read Uncommitted: Allows dirty reads
• Read Committed: Does not allow dirty reads
• Repeatable Read: If a row is read twice in the same transaciton, result will always be the same
• Serializable: Performs all transactions in a sequence

The different levels have different performance characteristics in a multi threaded application. I think if you understand the dirty reads concept you will be able to select a good option.

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Example when a dirty read can occur

  thread 1   thread 2||
    write(x)||||        read(x)||
    rollback    |
      v         v 
           value (x) is now dirty (incorrect)

So a sane default (if such can be claimed) could be Read Comitted, which only lets you read values which have already been comitted by other running transactions, in combination with an isolation level of Required. Then you can work from there if you application has other needs.

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A practical example where a new transaction will always be created when entering the provideService routine and completed when leaving.

publicclassFooService{privateRepository repo1;privateRepository repo2;@Transactional(propagation=Propagation.REQUIRES_NEW)publicvoid provideService(){
        repo1.retrieveFoo();
        repo2.retrieveFoo();}}

Had we used Required instead the transaction will remain open if the transaction was already open when entering the routine. Note also that the result of a rollback could be different as several executions could take part in the same transaction.

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We can easily verify the behaviour with a test and see how results differ with propagation levels

@RunWith(SpringJUnit4ClassRunner.class)@ContextConfiguration(locations="classpath:/fooService.xml")publicclassFooServiceTests{private@AutowiredTransactionManager;private@AutowiredFooService fooService;@Testpublicvoid testProvideService(){TransactionStatus status = transactionManager.getTransaction(newDefaultTransactionDefinition());
        fooService.provideService();
        transactionManager.rollback(status);// assert repository values are unchanged ... }