Spring in Action(3rd edition) Learning Notes

The bean below will be wired by its default constructor:

<bean id="hello" class="ini.kelvin.spring.Hello" />

But if the bean definition does not have a default constructor:

<bean id="hello" class="ini.kelvin.spring.Hello">
    <constructor-arg value="someValue" />
    <constructor-arg ref="beanId" />
<bean>

The example above will use the constructor which has two args, 1st one is basic type(err.. String should also be considered as "basic type"), 2nd one is an instance of an object, so it should be defined with <bean> tag and then the ref is the reference of that bean, whose id is beanId.

Some definition of classes may not have a visible constructor, an example is the singleton bean definition, as below:

public class Singleton {
    private Singleton() {}

    public static Singleton getInstance() {
        return SingletonHolder.instance;
    }

    private static class SingletonHolder {
        private static Singleton instance = new Singleton();
    }
}

Under this condition, we can use following to construct the bean:

<bean id="singleton"
      class="Singleton"
      factory-method="getInstance" />

The definition of bean scope is as following:

Specify attributes init-method and destroy-method in a bean definition, the method specified by init-method will be invoked after the bean is constructed, and the method specified by destroy-method will be invoked before the bean is removed from the container.

<bean id="xxx"
      class="xxx.Resource"
      init-method="allocateResource"
      destroy-method="freeResource" />

As the example shows, the required resources will be allocated after constructed, and the resources will be freed before bean removed.

<property> is like <constructor-arg>, example:

<bean id="hello" class="ini.kelvin.spring.Hello">
    <property name="stringProperty" value="someValue" />
    <property name="objectProperty" ref="beanId" />
<bean>

In the example above, the method setStringProperty will be invoked to inject the value someValue, and setObjectProperty will be invoked to set the related property to the bean referenced by beanId.

NOTICE: the attribute name of property should be related to the set method, not the field name, e.g. field name is realProperty, but the set method is setFakeProperty, so the name should be fakeProperty.

In the example above, if there are several property, and all of them have ref to "beanId", then the bean with id "beanId" will be shared by all of them. The example below will create a standalone InnerBean, only available for bean "hello":

<bean id="hello" class="ini.kelvin.spring.Hello">
    <property name="stringProperty" value="someValue" />
    <property name="objectProperty">
        <bean class="package.path.InnerBean" />
    </property>
<bean>

Elements for collection wiring:

An example of <list> (<set> is similar):

<bean ...>
    <property name="countries">
        <list>
            <ref bean="China" />
            <ref bean="USA" />
            ...
        </list>
    </property>
</bean>

An example of <map>:

<bean ...>
    <property name="keyValuePair">
        <map>
            <entry key="key1" value-ref="Obj1" />
            <entry key="key2" value-ref="Obj2" />
            ...
        </map>
    </property>
</bean>

The <entry> element contains a key value pair, both key and value could be a primitive value or a reference to another bean, when key/value is a primitive value, <key> / <value> should be used; when key/value referenced to other bean, <key-ref> / <value-ref> shoud be used.

An example of <props>:

<bean ...>
    <property name="personalInfo">
        <props>
            <prop key="name">Kelvin Hu</prop>
            <prop key="email">ini.kelvin@gmail.com</prop>
            ...
        </props>
    </property>
</bean>

Example:

<property ...>
    <null />
</property>

This section is omitted.

• byName : search beans, if a bean's id matches the property name, then it will be wired into this property
• byType : search beans, search criteria is class type
• constructor : search beans, the beans which can satisfy the bean's constructor wil be used to construct the bean
• autodetect : try autowiring by constructor first, if no suitable constructor or bean is found, then try autowiring by type.

By default annotation wiring is turned off, use the following line to turn it on:

<context:annotation-config />

Autowiring annotations:

1. @Autowired / @Qualifier
2. @Inject / @Named / @Qualifier
3. @Value (for primitive types, useful while using with SpEL)
4. @Resource

<context:annotation-config> is only used for automatically wiring, but <context:component-scan> will used for both autowiring, but also autodiscovery, as below:

<beans ...>
    <context:component-scan base-package="com.mycompany.xxx">
    </context:component-scan>
</beans>

Annotations can be used:

1. @Component : general-purpose annotation, indicating that the class is a Spring component
2. @Controller : defines a Spring MVC controller
3. @Repository : defines a data repository
4. @Service : defines a service
5. any customized annotation annotated with @Component

<context:component-scan> can include <context:include-filter> and <context:exclude-filter> to tweak component scanning. (this section is omitted, since it is not used so much)

not used so often, omitted

A typical configuration:

<bean id="aspectBean" class="package.AspectBean" />
...
<aop:config>
    <aop:aspect ref="aspectBean">
        <aop:pointcut id="aPointCut"
            expression="execution(* package.class.method(..))" />
        <aop:before pointcut-ref="aPointCut" method="aspectMethod" />
    </aop:aspect>
</aop:config>

For the example above, the class definition package.AspectBean should have a method named aspectMethod, which will be invoked before the execution of package.class.method().

But, for the type <aop:around>, it is not that simple like the example above. The XML configuration of this type is similar to <aop:before>, but the method aspectMethod defined in package.AspectBean should have the following prototype:

public void aspectMethod(ProceedingJoinPoint joinPoint)

and the invocation of joinPoint.proceed() will invoke the advised method, so code blocks can be put before and after this invocation, which can implement the effect of "around".

omitted