In most application scenarios, most beans in the container are singletons. When a singleton bean needs to collaborate with another singleton bean or a non-singleton bean, it typically requires a shared object. This is where Method Injection comes into play.
前言
我们先探一探官方文档关于Method Injection的章节是怎么说的：
In most application scenarios, most beans in the container are singletons. When a singleton bean needs to collaborate with another singleton bean or a non-singleton bean needs to collaborate with another non-singleton bean, you typically handle the dependency by defining one bean as a property of the other. A problem arises when the bean lifecycles are different. Suppose singleton bean A needs to use non-singleton (prototype) bean B, perhaps on each method invocation on A. The container creates the singleton bean A only once, and thus only gets one opportunity to set the properties. The container cannot provide bean A with a new instance of bean B every time one is needed.
官方文档首先也提到了一个解决方案就是把A弄成容器的Aware( make bean A aware of the container),也就是实现ApplicationContextAware接口。
A solution is to forego some inversion of control. You can make bean A aware of the container by implementing the ApplicationContextAware interface, and by making a getBean("B") call to the container ask for (a typically new) bean B instance every time bean A needs it. The following example shows this approach
文档里随后就提供了一个示例来说明这个解决方案如何做。
// a class that uses a stateful Command-style class to perform some processing
package fiona.apple;
// Spring-API imports
import org.springframework.beans.BeansException;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
public class CommandManager implements ApplicationContextAware {
private ApplicationContext applicationContext;
public Object process(Map commandState) {
// grab a new instance of the appropriate Command
Command command = createCommand();
// set the state on the (hopefully brand new) Command instance
command.setState(commandState);
return command.execute();
}
protected Command createCommand() {
// notice the Spring API dependency!
return this.applicationContext.getBean("command", Command.class);
}
public void setApplicationContext(
ApplicationContext applicationContext) throws BeansException {
this.applicationContext = applicationContext;
}
}
虽然解决了一开始提出的问题，但是Spring随后就说到：
The preceding is not desirable, because the business code is aware of and coupled to the Spring Framework. Method Injection, a somewhat advanced feature of the Spring IoC container, lets you handle this use case cleanly.
Lookup method injection is the ability of the container to override methods on container-managed beans and return the lookup result for another named bean in the container. The lookup typically involves a prototype bean, as in the scenario described in the preceding section. The Spring Framework implements this method injection by using bytecode generation from the CGLIB library to dynamically generate a subclass that overrides the method.
For this dynamic subclassing to work, the class that the Spring bean container subclasses cannot be final, and the method to be overridden cannot be final, either.
Unit-testing a class that has an abstract method requires you to subclass the class yourself and to supply a stub implementation of the abstract method.
Concrete methods are also necessary for component scanning, which requires concrete classes to pick up.
A further key limitation is that lookup methods do not work with factory methods and in particular not with @Bean methods in configuration classes, since, in that case, the container is not in charge of creating the instance and therefore cannot create a runtime-generated subclass on the fly.
In the case of the CommandManager class in the previous code snippet, the Spring container dynamically overrides the implementation of the createCommand() method. The CommandManager class does not have any Spring dependencies, as the reworked example shows.
Xml配置lookup-method
public abstract class CommandManager {
public Object process(Object commandState) {
// grab a new instance of the appropriate Command interface
Command command = createCommand();
// set the state on the (hopefully brand new) Command instance
command.setState(commandState);
return command.execute();
}
// okay... but where is the implementation of this method?
protected abstract Command createCommand();
}
Another way of accessing differently scoped target beans is an ObjectFactory/ Provider injection point. See Scoped Beans as Dependencies。You may also find the ServiceLocatorFactoryBean (in the org.springframework.beans.factory.config package) to be useful.
In most application scenarios, most beans in the container are singletons. When a singleton bean needs to collaborate with another singleton bean or a non-singleton bean, it typically requires a shared object. This is where Method Injection comes into play.
前言
我们先探一探官方文档关于Method Injection的章节是怎么说的：
In most application scenarios, most beans in the container are singletons. When a singleton bean needs to collaborate with another singleton bean or a non-singleton bean needs to collaborate with another non-singleton bean, you typically handle the dependency by defining one bean as a property of the other. A problem arises when the bean lifecycles are different. Suppose singleton bean A needs to use non-singleton (prototype) bean B, perhaps on each method invocation on A. The container creates the singleton bean A only once, and thus only gets one opportunity to set the properties. The container cannot provide bean A with a new instance of bean B every time one is needed.
官方文档首先也提到了一个解决方案就是把A弄成容器的Aware( make bean A aware of the container),也就是实现ApplicationContextAware接口。
A solution is to forego some inversion of control. You can make bean A aware of the container by implementing the ApplicationContextAware interface, and by making a getBean("B") call to the container ask for (a typically new) bean B instance every time bean A needs it. The following example shows this approach
文档里随后就提供了一个示例来说明这个解决方案如何做。
// a class that uses a stateful Command-style class to perform some processing
package fiona.apple;
// Spring-API imports
import org.springframework.beans.BeansException;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
public class CommandManager implements ApplicationContextAware {
private ApplicationContext applicationContext;
public Object process(Map commandState) {
// grab a new instance of the appropriate Command
Command command = createCommand();
// set the state on the (hopefully brand new) Command instance
command.setState(commandState);
return command.execute();
}
protected Command createCommand() {
// notice the Spring API dependency!
return this.applicationContext.getBean("command", Command.class);
}
public void setApplicationContext(
ApplicationContext applicationContext) throws BeansException {
this.applicationContext = applicationContext;
}
}
虽然解决了一开始提出的问题，但是Spring随后就说到：
The preceding is not desirable, because the business code is aware of and coupled to the Spring Framework. Method Injection, a somewhat advanced feature of the Spring IoC container, lets you handle this use case cleanly.
Lookup method injection is the ability of the container to override methods on container-managed beans and return the lookup result for another named bean in the container. The lookup typically involves a prototype bean, as in the scenario described in the preceding section. The Spring Framework implements this method injection by using bytecode generation from the CGLIB library to dynamically generate a subclass that overrides the method.
For this dynamic subclassing to work, the class that the Spring bean container subclasses cannot be final, and the method to be overridden cannot be final, either.
Unit-testing a class that has an abstract method requires you to subclass the class yourself and to supply a stub implementation of the abstract method.
Concrete methods are also necessary for component scanning, which requires concrete classes to pick up.
A further key limitation is that lookup methods do not work with factory methods and in particular not with @Bean methods in configuration classes, since, in that case, the container is not in charge of creating the instance and therefore cannot create a runtime-generated subclass on the fly.
In the case of the CommandManager class in the previous code snippet, the Spring container dynamically overrides the implementation of the createCommand() method. The CommandManager class does not have any Spring dependencies, as the reworked example shows.
Xml配置lookup-method
public abstract class CommandManager {
public Object process(Object commandState) {
// grab a new instance of the appropriate Command interface
Command command = createCommand();
// set the state on the (hopefully brand new) Command instance
command.setState(commandState);
return command.execute();
}
// okay... but where is the implementation of this method?
protected abstract Command createCommand();
}
Another way of accessing differently scoped target beans is an ObjectFactory/ Provider injection point. See Scoped Beans as Dependencies。You may also find the ServiceLocatorFactoryBean (in the org.springframework.beans.factory.config package) to be useful.
