基于构造器&＃xff08;或静态工厂方法&＃xff09;的依赖注入

基于构造器的注入&＃xff1a;容器通过调用指定参数的构造器完成bean的实例化&＃xff0c;并且通过构造器的参数完成依赖注入。&＃xff08;静态工厂方法与构造器的行为基本一致&＃xff09;

举例&＃xff1a;

public class SimpleMovieLister {// the SimpleMovieLister has a dependency on a MovieFinderprivate MovieFinder movieFinder;// a constructor so that the Spring container can inject a MovieFinderpublic SimpleMovieLister(MovieFinder movieFinder) {this.movieFinder &＃61; movieFinder;}// business logic that actually uses the injected MovieFinder is omitted...
}


构造器参数解析&＃xff1a;容器调用“哪一个”构造器。

策略&＃xff1a;通过类型匹配。通过构造器注入时&＃xff0c;根据传入的参数的类型&＃xff0c;匹配构造器&＃xff0c;如果找不到&＃xff0c;就会报错。

Java代码&＃xff1a;

package SpringTest.ConBasedDI;public class TestConDI {public TestConDI(ConArgOne arg1, ConArgOne arg2) {System.out.println("TestConDI(ConArgOne, ConArgOne)");}public TestConDI(ConArgOne arg1, ConArgTwo arg2){System.out.println("TestConDI(ConArgOne, ConArgTwo)");}public TestConDI(ConArgTwo arg1, ConArgOne arg2) {System.out.println("TestConDI(ConArgTwo, ConArgOne)"); }
}


当启动容器时&＃xff0c;运行结果&＃xff1a;

匹配时&＃xff0c;根据多个constructor-arg子标签指定多个参数类型&＃xff0c;子标签的顺序匹配构造器参数的顺序。

使用index属性调整顺序&＃xff0c;如果没有用使用index属性&＃xff0c;则默认是按照标签排列的顺序进行匹配&＃xff0c;如果设置了index属性&＃xff0c;则按照index值&＃xff08;从0开始&＃xff09;进行匹配

相当于

参数继承问题

对于上面这个例子&＃xff0c;没有让ConArgOne和ConArgTwo具有继承关系。

如果让ConArgTwo继承ConArgOne呢&＃xff1f;

运行结果没有变化。

请注意这个配置

继承之后TestConDI(ConArgOne, ConArgOne) 和 TestConDI(ConArgOne, ConArgTwo)都符合这个配置&＃xff0c;但还是调用了TestConDI(ConArgOne, ConArgTwo)。

如果有多个选择&＃xff0c;容器会帮我们匹配类型更精确的。

确定简单值的类型。

对于引用类型&＃xff0c;匹配构造器的时候根据引用的bean类型即可。但是对于简单类型来说&＃xff0c;并不能确定它的类型。

举个例子&＃xff1a;

package examples;public class ExampleBean {// Number of years to calculate the Ultimate Answerprivate int years;// The Answer to Life, the Universe, and Everythingprivate String ultimateAnswer;public ExampleBean(int years, String ultimateAnswer) {this.years &＃61; years;this.ultimateAnswer &＃61; ultimateAnswer;}
}


按照上面的配置方法&＃xff0c;可以这样配置。

可以看到&＃xff0c;简单的使用value属性是不能确定类型的。运行可能并不会出错

使用type属性指定类型&＃xff1a;

使用name属性指定参数名&＃xff1a;

举例&＃xff08;基于构造器注入 &＃43; 基于静态工厂方法注入&＃xff09;

基于构造器注入



public class ExampleBean {private AnotherBean beanOne;private YetAnotherBean beanTwo;private int i;public ExampleBean(AnotherBean anotherBean, YetAnotherBean yetAnotherBean, int i) {this.beanOne &＃61; anotherBean;this.beanTwo &＃61; yetAnotherBean;this.i &＃61; i;}
}


基于静态工厂方法注入



public class ExampleBean {// a private constructorprivate ExampleBean(...) {...}// a static factory method; the arguments to this method can be// considered the dependencies of the bean that is returned,// regardless of how those arguments are actually used.public static ExampleBean createInstance (AnotherBean anotherBean, YetAnotherBean yetAnotherBean, int i) {ExampleBean eb &＃61; new ExampleBean (...);// some other operations...return eb;}
}