在Spring的生态中，借助@Autowired注解来实现依赖注入，可以说是非常普遍的事情了，如果让我们自定义一个注解，也实现类似的功能，那么我们可以怎么做呢?

本文介绍如何实现一个自定义的@Autowired，实现依赖服务注入

主要知识点:

• BeanPostProcessor
• 代理类创建

I. 项目环境

本项目借助SpringBoot 2.2.1.RELEASE + maven 3.5.3 + IDEA进行开发

下面是核心的pom.xml（源码可以再文末获取）

<!-- 这个依赖是干嘛的，后文会介绍 -->
<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-configuration-processor</artifactId>
    </dependency>
</dependencies>

II. 实现姿势

1. 代理封装类

借助Spring的Enhance来实现代理类生成，比如一个基础的工具类如下，用于自定义注入的增强

public class ProxyUtil {
    public static <T> T newProxyInstance(Class<?> targetClass, InvocationHandler invocationHandler,
                                         ProxyUtil.CallbackFilter filter) {
        if (targetClass == null) {
            return null;
        } else {
            Enhancer enhancer = new Enhancer();
            enhancer.setSuperclass(targetClass);
            enhancer.setUseCache(true);
            enhancer.setCallback(new ProxyUtil.SimpleMethodInterceptor(invocationHandler, filter));
            // 无参构造方法  
            return (T) enhancer.create();
        }
    }

public interface CallbackFilter {
        boolean accept(Method var1);
    }

private static class SimpleMethodInterceptor implements MethodInterceptor, Serializable {
        private transient InvocationHandler invocationHandler;
        private transient ProxyUtil.CallbackFilter filter;

public SimpleMethodInterceptor(InvocationHandler invocationHandler, ProxyUtil.CallbackFilter filter) {
            this.invocationHandler = invocationHandler;
            this.filter = filter;
        }

@Override
        public Object intercept(Object o, Method method, Object[] objects, MethodProxy methodProxy) throws Throwable {
            return this.filter.accept(method) ? this.invocationHandler.invoke(o, method, objects) : methodProxy.invokeSuper(o, objects);
        }
    }
}

2. 自定义注解

参照@Autowired的定义，实现一个自定义的注解（缩减版）

@Target({ElementType.METHOD, ElementType.FIELD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface AutoInject {
}

3. 自定义注入

实现BeanPostProcessor，在bean初始化之后，扫描field/method，为了做一个区分，下面创建一个代理类，注入依赖

@Component
public class AutoInjectPostProcessor implements BeanPostProcessor {

private ApplicationContext applicationContext;

public AutoInjectPostProcessor(ApplicationContext applicationContext) {
        this.applicationContext = applicationContext;
    }

@Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        Class<?> clazz = bean.getClass();
        do {
            for (final Field field : clazz.getDeclaredFields()) {
                final AutoInject annotation = AnnotationUtils.findAnnotation(field, AutoInject.class);
                if (annotation != null) {
                    ReflectionUtils.makeAccessible(field);
                    ReflectionUtils.setField(field, bean, processInjectionPoint(field.getType()));
                }
            }
            for (final Method method : clazz.getDeclaredMethods()) {
                final AutoInject annotation = AnnotationUtils.findAnnotation(method, AutoInject.class);
                if (annotation != null) {
                    final Class<?>[] paramTypes = method.getParameterTypes();
                    if (paramTypes.length != 1) {
                        throw new BeanDefinitionStoreException(
                                "Method " + method + " doesn't have exactly one parameter.");
                    }
                    ReflectionUtils.makeAccessible(method);
                    ReflectionUtils.invokeMethod(method, bean,
                            processInjectionPoint(paramTypes[0]));
                }
            }
            clazz = clazz.getSuperclass();
        } while (clazz != null);
        return bean;
    }

// 创建代理类，在具体方法执行前后输出一个日志
    protected <T> T processInjectionPoint(final Class<T> injectionType) {
        return ProxyUtil.newProxyInstance(injectionType, new InvocationHandler() {
            @Override
            public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                System.out.println("do before " + method.getName() + " | " + Thread.currentThread());
                try {
                    Object obj = applicationContext.getBean(injectionType);
                    return method.invoke(obj, args);
                } finally {
                    System.out.println("do after " + method.getName() + " | " + Thread.currentThread());
                }
            }
        }, new ProxyUtil.CallbackFilter() {
            @Override
            public boolean accept(Method var1) {
                return true;
            }
        });
    }
}

接下来验证一下自定义注入方式

@Component
public class DemoService {

public int calculate(int a, int b) {
        doBefore();
        return a + b;
    }

private void doBefore() {
        System.out.println("-------- inner ----------: " + Thread.currentThread());
    }
}

@Component
public class DemoService2 {

public int calculate(int a, int b) {
        doBefore();
        return a + b;
    }

private void doBefore() {
        System.out.println("-------- inner ----------: " + Thread.currentThread());
    }
}

@Service
public class RestService {
    @AutoInject
    private DemoService demoService;

private DemoService2 demoService2;

@AutoInject
    public void setDemoService2(DemoService2 demoService2) {
        this.demoService2 = demoService2;
    }

public void test() {
        int ans = demoService.calculate(10, 20);
        System.out.println(ans);

ans = demoService2.calculate(11, 22);
        System.out.println(ans);
    }
}

执行完毕之后，输出日志如

do before calculate | Thread[main,5,main]
-------- inner ----------: Thread[main,5,main]
do after calculate | Thread[main,5,main]
30

do before calculate | Thread[main,5,main]
-------- inner ----------: Thread[main,5,main]
do after calculate | Thread[main,5,main]
33

II. 其他

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尽信书则不如，以上内容，纯属一家之言，因个人能力有限，难免有疏漏和错误之处，如发现bug或者有更好的建议，欢迎批评指正，不吝感激

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