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10.2.4 The DAO elements
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Once the transaction manager has been chosen and configured, the DAO elements can be added to the DAO context to define the DAO interfaces and implementations that your context will make available for your application. The <dao> element only has two properties: interface and implementation.
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Just because the attribute name interface is used to identify the DAO implementation, you do not actually need to use an interface. In our previous example, both of the attributes (interface and implementation) could have been the fully qualified class name of the implementation. While this may seem like an easy way to reduce some code, it is strongly discouraged, because it essentially eliminates the value added by the DAO layer that is, the separation of the interface from the implementation. As for the code savings, nearly all IDEs now provide refactoring tools to allow you to extract an interface from an implementation, meaning that you are able to write and test your DAO implementation without creating the interface and then create the interface with a few mouse clicks.
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The interface property is used to identify the DAO in the DAO map, and is generally used in the following way:
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<dao interface="com.mycompany.system.dao.AccountDao" implementation= "com.mycompany.system.dao.impl.AccountDaoImpl"/>
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iBATIS data access objects
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Let s look at an example of this. Assume a class relationship like the one described earlier in figure 10.2. In this diagram, there is an AccountDao interface and an AccountDaoImpl class that implements that interface. To use the DaoManager to get the DAO, use the following code:
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AccountDao accountDao = (AccountDao) daoManager.getDao(AccountDao.class);
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In this line of code, we declare our AccountDao variable, request it from the DaoManager instance using the interface name, and then cast it to the AccountDao interface, because the DAO manager simply returns Object. In the previous version of the DAO, it was also possible to pass in a String instead of an interface class. In version 2.x, that functionality was dropped, because it eliminated a potential point of failure. By forcing the use of a class name for identifying DAO implementations, you ensure that misspellings can be avoided in the Java environment, because if the interface name is misspelled, the code will not compile. Early failure is a good thing. So, now that you have a solid foundational understanding of what you can do with the iBATIS DAO framework, we can start looking at other more advanced uses of it.
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10.3 Configuration tips
Although the DAO configuration looks very simple on the surface, it still offers a great deal of flexibility. By creatively configuring the DAO manager, you can accomplish some pretty sophisticated approaches to common problems. Let s look at a few of them.
10.3.1 Multiple servers
As mentioned earlier, it is not uncommon in most development shops to have different servers for their development, QC testing, UA testing, and production environments. In these cases, it is very useful to be able to remove the environment-specific information from the dao.xml file and put it into an external file. The properties element was created to accomplish just that sort of thing. Listing 10.3 is a sample dao.xml file that uses this technique to make the JDBC settings external to the dao.xml file.
Configuration tips
Listing 10.3 A sample dao.xml with JDBC settings inserted with a <properties /> element
< xml version="1.0" encoding="UTF-8" > <!DOCTYPE daoConfig PUBLIC "-//ibatis.apache.org//DTD DAO Configuration 2.0//EN" "http://ibatis.apache.org/dtd/dao-2.dtd"> <daoConfig> <properties resource="server.properties"/> <context> <transactionManager type="JDBC"> <property name="DataSource" value="SIMPLE"/> <property name="JDBC.Driver" value="${jdbcDriver}" /> <property name="JDBC.ConnectionURL" value="${jdbcUrl}" /> <property name="JDBC.Username" value="${jdbcUser}" /> <property name="JDBC.Password" value="${jdbcPassword}" /> <property name="JDBC.DefaultAutoCommit" value="${jdbcAutoCommit}" /> </transactionManager> <dao interface="..." implementation="..."/> </context> </daoConfig>
In this example, all of the property values are stored in a file named server.properties that is to be loaded from the root of the classpath. This is an approach that we like to use, because all of the files can be kept under version control, and the different versions of the properties files can be named in a way that identifies them based on the environment (i.e., server-production.properties, server-user.properties, etc.) so that the build procedure can automatically copy the correct version to the correct location. This approach also works well in more sensitive environments, where the configuration settings are considered to be more secure by not having them under version control. In those environments, it makes manual configuration simpler, because the configuration file that changes based on the environment is always the same file.
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