Spring框架第二篇之Bean的装配

一、默认装配方式

代码通过getBean();方式从容器中获取指定的Bean实例，容器首先会调用Bean类的无参构造器，创建空值的实例对象。

举例：

首先我在applicationContext.xml配置文件中配置了一个bean：

<?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">

    <!-- 注册Service 这里相当于容器做了SomeServiceImpl myService = new SomeServiceImpl(); -->
    <bean id="myService" class="com.ietree.spring.basic.ioc.SomeServiceImpl"/>
    
</beans>

创建SomeServiceImpl对象，但需要注意的是该类的只具有带参构造函器，没有无参构造器：

package com.ietree.spring.basic.ioc;

/**
 * 实现类
 * 
 * @author Root
 */
public class SomeServiceImpl implements ISomeService {

    private int a;
    
    // 这里注释掉了无参构造函数，希望容器通过带参构造函数创建对象
//    public SomeServiceImpl() {
//        System.out.println("执行无参构造器，创建SomeServiceImpl对象");
//    }

    public SomeServiceImpl(int a) {
        this.a = a;
    }

    @Override
    public void doSomeThing() {
        System.out.println("执行doSomeThing()方法...");
    }

}

测试：

@Test
public void testConstructor() {

    // 创建容器对象，加载Spring配置文件
    // ClassPathXmlApplicationContext会从类路径下查找配置文件
    ApplicationContext ac = new ClassPathXmlApplicationContext("applicationContext.xml");

    ISomeService service = (ISomeService) ac.getBean("myService");
    service.doSomeThing();
}

此时程序会报以下的错误：

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'myService' defined in class path resource [applicationContext.xml]: Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.ietree.spring.basic.ioc.SomeServiceImpl]: No default constructor found; nested exception is java.lang.NoSuchMethodException: com.ietree.spring.basic.ioc.SomeServiceImpl.<init>()
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:1155)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1099)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:513)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:761)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:867)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:543)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83)
    at com.ietree.spring.basic.test.MyTest.testConstrutor(MyTest.java:67)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:678)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)
Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.ietree.spring.basic.ioc.SomeServiceImpl]: No default constructor found; nested exception is java.lang.NoSuchMethodException: com.ietree.spring.basic.ioc.SomeServiceImpl.<init>()
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:85)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:1147)
    ... 36 more
Caused by: java.lang.NoSuchMethodException: com.ietree.spring.basic.ioc.SomeServiceImpl.<init>()
    at java.lang.Class.getConstructor0(Unknown Source)
    at java.lang.Class.getDeclaredConstructor(Unknown Source)
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:80)
    ... 37 more

解析：这里的错误报的很明显，没有发现默认的构造器。

修改：为该类加上无参构造器：

package com.ietree.spring.basic.ioc;

/**
 * 实现类
 * 
 * @author Root
 */
public class SomeServiceImpl implements ISomeService {

    private int a;
    
    public SomeServiceImpl() {
        System.out.println("执行无参构造器，创建SomeServiceImpl对象");
    }

    public SomeServiceImpl(int a) {
        this.a = a;
    }

    @Override
    public void doSomeThing() {
        System.out.println("执行doSomeThing()方法...");
    }

}

此时，再次运行测试用例，会发现运行成功。

结论：Spring容器实际上是使用了类的反射机制，会首先调用Bean类的无参构造器创建实例对象。

二、动态工厂Bean

创建SomeServiceImpl类：

package com.ietree.spring.basic.ioc;

/**
 * 实现类
 * 
 * @author Root
 */
public class SomeServiceImpl implements ISomeService {

    public SomeServiceImpl() {
        System.out.println("执行无参构造器，创建SomeServiceImpl对象");
    }

    @Override
    public void doSomeThing() {
        System.out.println("执行doSomeThing()方法...");
    }

}

创建工厂类ServiceFactory：

package com.ietree.spring.basic.ioc;

/**
 * 工厂类
 * 
 * @author Root
 */
public class ServiceFactory {

    public ISomeService getSomeService() {
        return new SomeServiceImpl();
    }

}

使用动态工厂方式获取Bean对象，配置如下：

<?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">

    <!-- 注册动态工厂 -->
    <bean id="factory" class="com.ietree.spring.basic.ioc.ServiceFactory"/>
    
    <!-- 注册Service：动态工厂Bean -->
    <bean id="myService" factory-bean="factory" factory-method="getSomeService"/>
    
</beans>

在这里并没有注册SomeServiceImpl类，而是通过ServiceFactory工厂的getSomeService方法获取的。

测试：

@Test
public void testFactory1() {
        
    ApplicationContext ac = new ClassPathXmlApplicationContext("applicationContext.xml");
    ISomeService service = (ISomeService) ac.getBean("myService");
    service.doSomeThing();
}

运行成功。

三、静态工厂Bean

静态工厂和动态工厂不同的是，静态工厂中使用的是静态方法创建对象，如：

package com.ietree.spring.basic.ioc;

/**
 * 工厂类
 * 
 * @author Root
 */
public class ServiceFactory {
    
    // 使用静态方法创建对象
    public static ISomeService getSomeService() {
        return new SomeServiceImpl();
    }

}

对应的配置文件修改如下：

<?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">

    <!-- 注册Service：静态工厂Bean -->
    <bean id="myService" class="com.ietree.spring.basic.ioc.ServiceFactory" factory-method="getSomeService"/>
    
</beans>

测试：

@Test
public void testFactory1() {
        
    ApplicationContext ac = new ClassPathXmlApplicationContext("applicationContext.xml");
    ISomeService service = (ISomeService) ac.getBean("myService");
    service.doSomeThing();
}

成功创建SomeServiceImpl对象。

四、容器中的Bean的作用域

Bean的作用域（scope）分为四种，分别是singleton、prototype、request、session。

scope：
　　singleton（默认）：单例模式，其对象的创建时机是在Spring容器初始化时创建，是默认值
　　prototype：原型模式，其对象的创建时机不是在Spring容器初始化时创建，而是在代码中真正访问时才创建，每次使用getBean方法获取的同一个<bean/>的实例都是一个新的实例
　　request：对于每次HTTP请求，都将会产生一个不同的Bean实例
　　session：对于每个不同的HTTP session，都将会产生一个不同的Bean实例

验证：

首先配置作用域为singleton：

<bean id="myService" class="com.ietree.spring.basic.ioc.SomeServiceImpl" scope="singleton"/>

测试：

@Test
public void test05() {
        
    ApplicationContext ac = new ClassPathXmlApplicationContext("applicationContext.xml");
        
    ISomeService service1 = (ISomeService) ac.getBean("myService");
    ISomeService service2 = (ISomeService) ac.getBean("myService");
    System.out.println("service1 = service2吗？" + (service1 == service2));
}

程序输出：

调用无参构造器
service1 = service2吗？true

结论：当作用域为singleton单例模式时，只会创建一个对象实例，并且对象是在Spring容器初始化时创建。

同样，当配置为prototype原型模式时：

<bean id="myService" class="com.ietree.spring.basic.ioc.SomeServiceImpl" scope="prototype"/>

程序输出：

调用无参构造器
调用无参构造器
service1 = service2吗？false

结论：构造器被调用了两次，说明创建的service1和service2不是同一个对象，并且对象是在被使用到时才创建的。

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Spring框架第二篇之Bean的装配

一、默认装配方式

二、动态工厂Bean

三、静态工厂Bean

四、容器中的Bean的作用域

五、Bean后处理器

六、定制Bean的生命周期

七、<bean/>标签的id属性与name属性