Bean实例化的三种方式

具体代码如下,本实例是用Maven写的,三种方式均需要相同的pom.xml配置文件。
pom.xml的配置文件如下

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.zh</groupId>
    <artifactId>spring-test</artifactId>
    <version>1.0.0-SNAPSHOT</version>
    <packaging>jar</packaging>

    <dependencies>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>4.1.6.RELEASE</version>
        </dependency>
    </dependencies>

</project>

1. 使用类的无参构造创建(推荐)

spring-context.xml的配置文件

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

   <bean id="user" class="com.zh.spring.test.entity.User"></bean>
</beans>

User.java实体类

public class User {
    private String username;
    private String height;

    public User() {
    }

    public String getUsername() {

        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getHeight() {
        return height;
    }

    public void setHeight(String height) {
        this.height = height;
    }

}

UserTest.java用户测试类

public class UserTest {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("spring-context.xml");
        User user = (User) context.getBean("user");
        System.out.println(user);

    }
}

打印结果如下：
com.zh.spring.test.entity.User@d2cc05a，因为User.java中没有重写toString()方法
注意：当User.java的实体类中将无参构造方法改为有参构造方法时(即User实体类中没有无参构造方法)，会报默认构造找不到异常No default constructor found;

2. 使用静态工厂方式创建

spring-context.xml的配置文件

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

   <bean id="bean2" class="com.zh.spring.test.BeanFactory" factory-method="getBean2"></bean>
</beans>

Bean2.java

public class Bean2 {
   public void add() {
       System.out.println("Bean2--------------");
   }

}

BeanFactory.Java 工厂类

public class BeanFactory {

    public static Bean2 getBean2() {
        return new Bean2();
    }
}

BeanTest.java

public class BeanTest {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("spring-context.xml");
        Bean2 bean2 = (Bean2) context.getBean("bean2");
        System.out.println(bean2);

    }
}

打印结果如下：
com.zh.spring.test.Bean2@7823a2f9，因为User.java中没有重写toString()方法

3. 使用实例工厂创建

spring-context.xml的配置文件

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
   <!--使用实例工厂创建对象-->
   <bean id="beanFactory" class="com.zh.spring.test.BeanFactory"></bean>
   <bean id="bean3" factory-bean="beanFactory" factory-method="getBean3"></bean>
</beans>

Bean3.java

public class Bean3 {
   public void add() {
       System.out.println("Bean3--------------");
   }
}

BeanFactory.java

public class BeanFactory {

    public Bean3 getBean3() {
        return new Bean3();
    }
}

BeanTest.java

public class BeanTest {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("spring-context.xml");
        Bean3 bean3 = (Bean3) context.getBean("bean3");
        System.out.println(bean3);

    }
}

打印结果如下：
com.zh.spring.test.Bean3@7823a2f9，因为User.java中没有重写toString()方法