Spring中创建对象的方式整理

本文主要是介绍Spring中创建对象的方式整理，对大家解决编程问题具有一定的参考价值，需要的程序猿们随着小编来一起学习吧！

在没有使用Spring时，开发者创建对象一般都是使用new/反射创建对象；Spring出来后简化了开发方式，它提供了一个IoC容器的实现，用于帮助开发者以依赖注入的方式管理对象之间的依赖关系，从而将开发者创建对象的所有权转移到IoC容器上，这使得Spring易于整合第三方模块，因此Spring更是一个分层的框架；

对于Spring创建对象的方式创建整理如下：

方式一：自定义BeanPostProcessor，生成代理对象

如实现InstantiationAwareBeanPostProcessor接口，其中InstantiationAwareBeanPostProcessor是一个BeanPostProcessor，它可用于处理bean实例创建前后的回调；

测试如下：

@Test
public void instantiationAwareBeanPostProcessorTest() {
    AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();

    GenericBeanDefinition definition1 = new GenericBeanDefinition();
    definition1.setBeanClass(DemoInstantiationAwareBPP.class);
    context.registerBeanDefinition("demoInstantiationAwareBPP", definition1);

    GenericBeanDefinition definition2 = new GenericBeanDefinition();
    definition2.setBeanClass(InstantiationDemo.class);
    context.registerBeanDefinition("instantiationDemo", definition2);

    context.refresh();
    InstantiationDemo bean = context.getBean(InstantiationDemo.class);
    bean.invoke();
}

public class DemoInstantiationAwareBPP implements InstantiationAwareBeanPostProcessor {
	private final static Log LOG = LogFactory.getLog(DemoInstantiationAwareBPP.class);

	@Override
	public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {
		LOG.info("beanName:" + beanName + "执行postProcessBeforeInstantiation方法");
		// 使用cglib生成代理对象
		if (beanClass == InstantiationDemo.class) {
			Enhancer enhancer = new Enhancer();
			enhancer.setSuperclass(beanClass);
			enhancer.setCallback((MethodInterceptor) (o, method, objects, methodProxy) -> {
				boolean objFlag = method.getDeclaringClass().getName().equals("java.lang.Object");
				if (!objFlag) {

					LOG.info("执行方法" + method + "前");
					Object rtn = methodProxy.invokeSuper(o, objects);
					LOG.info("执行方法" + method + "后");
					return rtn;
				} else {
					return methodProxy.invokeSuper(o, objects);
				}
			});
			InstantiationDemo bean = (InstantiationDemo) enhancer.create();
			LOG.info("创建代理对象:" + bean);
			return bean;
		}

		return null;
	}

	@Override
	public boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException {
		LOG.info("beanName:" + beanName + "执行postProcessAfterInstantiation方法");
		return false;
	}
}

public class InstantiationDemo {
	private final static Log LOG = LogFactory.getLog(InstantiationDemo.class);
	public void invoke() {
		LOG.info(this);
		LOG.info("InstantiationDemo invoke");
	}
}

注：

MethodProxy#invokeSuper是退出当前interceptor的处理,进入下一个callback处理；

MethodProxy#invoke则会继续回调该方法,如果传递给invoke的obj参数出错容易造成递归调用；

方式二：通过反射创建

AbstractAutowireCapableBeanFactory#doCreateBean为真正创建bean的逻辑，该方法是最复杂的，包含了调用构造函数，给bean的属性赋值，调用bean的初始化操作以及生成代理对象；

该方法会调用AbstractAutowireCapableBeanFactory#createBeanInstance实例化对象，如下图；

AbstractAutowireCapableBeanFactory#createBeanInstance根据@Autowried自动注入/调用无参构造器创建，进行相应的处理；

instantiateBean 调用无参数的构造器进行创建对象；

autowireConstructor @Autowired自动注入，调用构造器注入；

方式三：通过FactoryBean创建对象

AbstractBeanFactory#getObjectForBeanInstance

如果有实现了FactoryBean接口的实例，则从FactoryBean中获取实例；

测试如下：

@Test
public void mainTest13() {
    AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(
            MainConfig10.class);
 
    System.out.println("IOC容器创建完成...");
 
    // 调用FactoryBean#getBean方法,入参为Bean id,得到的对象为FactoryBean#getObjectType的对象
    // FactoryBean要获取工厂Bean本身,需要在id前加个&
    // org.springframework.beans.factory.support.AbstractBeanFactory.getObjectForBeanInstance
    Object factoryBean1 = context.getBean("demoFactoryBean");
    Object factoryBean2 = context.getBean("demoFactoryBean");
    System.out.println("factoryBean1==factoryBean2 :" + (factoryBean1 == factoryBean2));
    System.out.println("bean的类型:" + factoryBean1.getClass());
 
    Object factoryBean3 = context.getBean("&demoFactoryBean");
    System.out.println("bean的类型:" + factoryBean3.getClass());
}

@Configuration
public class MainConfig10 {
    @Bean
    public DemoFactoryBean demoFactoryBean() {
        return new DemoFactoryBean();
    }
}

public class DemoFactoryBean implements FactoryBean<Person> {
 
    /**
     * 是否单例进行控制
     * @return
     */
    @Override
    public boolean isSingleton() {
        return true;
    }
 
    /**
     * 返回对象，把对象放到容器中
     * @return
     * @throws Exception
     */
    @Override
    public Person getObject() throws Exception {
        return new Person();
    }
 
    /**
     * 返回对象类型
     * @return
     */
    @Override
    public Class<?> getObjectType() {
        return Person.class;
    }
}

方式四：通过设置BeanDefinition属性Supplier创建对象

AbstractAutowireCapableBeanFactory#createBeanInstance

获取bean的supplier；

AbstractAutowireCapableBeanFactory#obtainFromSupplier

通过Suupplier#get方法获取实例，此时获取实例不是使用反射；

测试如下：

@Test
public void supplierTest() {
    AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();

    GenericBeanDefinition definition = new GenericBeanDefinition();
    definition.setBeanClass(BeanC.class);
    definition.setInstanceSupplier((Supplier<BeanC>) BeanC::new);

    context.registerBeanDefinition("beanC", definition);
    context.refresh();
    Assertions.assertNotNull(context.getBean("beanC"));
}

public class BeanC {
	private final static Log LOG = LogFactory.getLog(BeanC.class);

	public BeanC() {
		LOG.info("BeanC constructor");
	}
}

方式五：通过设置BeanDefinition属性factoryMethod创建对象

AbstractAutowireCapableBeanFactory#createBeanInstance

createBeanInstance是实例化对象的过程，如果beanDefinition设置了factoryMethod，则从设置的factoryMethod实例化，而supplier属性的优先级高于factoryMethod，即beanDefinition同时存在supplier和factoryMethod属性时，优先执行supplier的实例化逻辑；

factoryMethod使用方式：往beanDefinition设置factoryMethod所属类的beanName和factoryMethodName的属性；

使用factoryMethod实例化对象的解析逻辑：ConstructorResolver#instantiateUsingFactoryMethod

SimpleInstantiationStrategy最终通过对factoryBean反射调用factoryMethod；

测试如下：

beanDefinition只有factoryMethod属性

public class FactoryDemo {
	private final static Log LOG = LogFactory.getLog(FactoryDemo.class);
	private int num;
	private String msg;

	public int getNum() {
		return num;
	}

	public void setNum(int num) {
		this.num = num;
	}

	public String getMsg() {
		return msg;
	}

	public void setMsg(String msg) {
		this.msg = msg;
	}

	public FactoryDemo() {
		LOG.info("FactoryDemo no params constructor");
	}

	public BeanDemo create() {
		BeanDemo beanDemo = new BeanDemo();
		LOG.info("invoke create without args,beanDemo:" + beanDemo);
		return beanDemo;
	}

	public BeanDemo create(int num, String msg) {
		BeanDemo beanDemo = new BeanDemo();
		beanDemo.setMsg(msg);
		beanDemo.setNum(num);
		LOG.info("invoke create with args,beanDemo:" + "[" + beanDemo + "]" + beanDemo.getMsg() + "," + beanDemo.getNum());
		return beanDemo;
	}
}

public class BeanDemo {
	private final static Log LOG = LogFactory.getLog(BeanDemo.class);

	private int num;
	private String msg;

	public BeanDemo() {
		LOG.info("BeanDemo constructor");
	}

	public int getNum() {
		return num;
	}

	public void setNum(int num) {
		this.num = num;
	}

	public String getMsg() {
		return msg;
	}

	public void setMsg(String msg) {
		this.msg = msg;
	}
}

调用无参的factoryMethod

@Test
public void factoryBeanNameTestWithoutArgs() {
    AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();
    context.register(FactoryDemo.class);

    GenericBeanDefinition definition = new GenericBeanDefinition();
    // 设置factoryBean
    definition.setFactoryBeanName("factoryDemo");
    definition.setFactoryMethodName("create");

    definition.setBeanClass(BeanDemo.class);
    context.registerBeanDefinition("beanDemo", definition);
    context.refresh();

    System.out.println(context.getBean(BeanDemo.class));
}

调用有参factoryMethod

@Test
public void factoryBeanNameWithArgsTest() {
    AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();
    context.register(FactoryDemo.class);

    GenericBeanDefinition definition = new GenericBeanDefinition();
    // 设置factoryBean
    definition.setFactoryBeanName("factoryDemo");
    definition.setFactoryMethodName("create");

    ConstructorArgumentValues argumentValues = new ConstructorArgumentValues();
    argumentValues.addGenericArgumentValue("test", "java.lang.String");
    argumentValues.addGenericArgumentValue(1, "int");
    definition.setConstructorArgumentValues(argumentValues);

    definition.setBeanClass(BeanDemo.class);
    context.registerBeanDefinition("beanDemo", definition);
    context.refresh();

    System.out.println(context.getBean(BeanDemo.class));
}

beanDefinition有factoryMethod属性和supplier属性

@Test
public void supplierOrderTest() {
    AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();
    context.register(FactoryDemo.class);

    GenericBeanDefinition definition = new GenericBeanDefinition();
    // 设置factoryBean
    definition.setFactoryBeanName("factoryDemo");
    definition.setFactoryMethodName("create");

    definition.setBeanClass(BeanDemo.class);
    context.registerBeanDefinition("beanDemo", definition);
    definition.setInstanceSupplier((Supplier<BeanDemo>) BeanDemo::new);

    context.refresh();
    Assertions.assertNotNull(context.getBean("beanDemo"));
}

此时优先执行supplier逻辑；

Spring创建对象方式的整理图

Spring实例化策略整理

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