EJB 3.1 @Startup @Singleton sequence

The annotation javax.ejb.Startup (@Startup) is used to mark an EJB so to make the EJB can be brought up during the application is running up. However, if you have multiple automatic loaded EJBs, then a question comes to you, how to make sure there loading sequence? Now, in EJB 3.1, you may use the annotation javax.ejb.DependsOn (@DependsOn) to achieve this. Please see the following example:

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@Singleton

public class PrimaryBean { ... }

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SecondaryBean depends on PrimaryBean:

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@Singleton

@DependsOn("PrimaryBean")

public class SecondaryBean { ... }

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If there is a third been needed to be automatically loaded, and you want the third been will be loaded after PrimaryBean and SecondaryBean have been loaded, so you need to:

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@Singleton

@DependsOn("PrimaryBean", "SecondaryBean")

public class ThirdBean { ... }

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In the case above, EJBs will be loaded in the order: 1. PrimaryBean; 2. SecondaryBean; 3. ThirdBean. If you just removed the dependence from SecondaryBean, the container will load either PrimaryBean or SecondaryBean firstly, then the ThirdBean.

 

Reference:

1. Initializing Singleton Session Beans (The Java EE 6 Tutorial) https://docs.oracle.com/cd/E19798-01/821-1841/gippq/index.html