Setter 注入（Setter Injection）

Setter 注入是通过类的公共 Setter 方法来实现依赖关系的配置。这种方式允许在对象实例化后灵活地更改依赖关系。

XML 配置文件 (app-config.xml)

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:p="http://www.springframework.org/schema/p"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
          http://www.springframework.org/schema/beans/spring-beans.xsd">

    <!-- 使用 p 命名空间简化 Setter 注入 -->
    <bean id="displayer" 
          class="com.example.spring.xml.ConsoleDataDisplayer"
          p:dataSource-ref="source"/>

    <bean id="source" 
          class="com.example.spring.xml.DefaultContentSource"/>
</beans>

Java 接口定义

package com.example.spring;

public interface ContentSource {
    String provideContent();
}

public interface DataDisplayer {
    void display();
    void setDataSource(ContentSource source);
    ContentSource getDataSource();
}

Java 类实现

package com.example.spring.xml;

import com.example.spring.ContentSource;

public class DefaultContentSource implements ContentSource {
    @Override
    public String provideContent() {
        return "Hello, Spring Setter Injection!";
    }
}

package com.example.spring.xml;

import com.example.spring.ContentSource;
import com.example.spring.DataDisplayer;

public class ConsoleDataDisplayer implements DataDisplayer {
    private ContentSource dataSource;

    @Override
    public void display() {
        if (dataSource == null) {
            throw new IllegalStateException("The dataSource property must be initialized.");
        }
        System.out.println(dataSource.provideContent());
    }

    @Override
    public void setDataSource(ContentSource source) {
        this.dataSource = source;
    }

    @Override
    public ContentSource getDataSource() {
        return this.dataSource;
    }
}

启动程序

package com.example.spring;

import org.springframework.context.support.GenericXmlApplicationContext;

public class SpringRunner {
    public static void main(String[] args) {
        GenericXmlApplicationContext context = new GenericXmlApplicationContext();
        context.load("classpath:app-config.xml");
        context.refresh();

        DataDisplayer displayer = context.getBean("displayer", DataDisplayer.class);
        displayer.display();
        
        context.close();
    }
}

构造函数注入（Constructor Injection）

构造函数注入确保了 Bean 在创建时就处于完整状态。Spring 支持通过参数索引、类型或名称来解决构造函数歧义。

XML 配置文件 (constructor-config.xml)

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:c="http://www.springframework.org/schema/c"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
       http://www.springframework.org/schema/beans/spring-beans.xsd">

    <!-- 使用 c 命名空间进行构造函数注入 -->
    <bean id="customSource" 
          class="com.example.spring.xml.ConfigurableContentSource"
          c:text="Dynamic message via constructor"/>

    <!-- 演示类型匹配解决构造函数重载歧义 -->
    <bean id="typeDemo"
          class="com.example.spring.xml.ConstructorTypeResolver"> 
        <constructor-arg type="int" value="1024"/>
    </bean>
</beans>

Java 类实现

package com.example.spring.xml;

import com.example.spring.ContentSource;

public class ConfigurableContentSource implements ContentSource {
    private final String text;

    public ConfigurableContentSource(String text) {
        this.text = text;
    }

    @Override
    public String provideContent() {
        return text;
    }
}

package com.example.spring.xml;

import org.springframework.context.support.GenericXmlApplicationContext;

public class ConstructorTypeResolver {
    private String val;

    public ConstructorTypeResolver(String val) {
        System.out.println("String constructor triggered");
        this.val = val;
    }

    public ConstructorTypeResolver(int val) {
        System.out.println("Integer constructor triggered");
        this.val = "Value: " + val;
    }

    @Override
    public String toString() {
        return val;
    }

    public static void main(String[] args) {
        GenericXmlApplicationContext ctx = new GenericXmlApplicationContext();
        ctx.load("classpath:constructor-config.xml");
        ctx.refresh();

        ConstructorTypeResolver resolver = ctx.getBean("typeDemo", ConstructorTypeResolver.class);
        System.out.println("Result: " + resolver);

        ContentSource source = ctx.getBean("customSource", ContentSource.class);
        System.out.println("Source output: " + source.provideContent());
        
        ctx.close();
    }
}