背景

写代码的时候，复制粘贴的时候，没注意到方法的属性，就导致了Controller里有了一个私有的方法，然后访问这个接口的时候就报了空指针异常，找了好久才找到是这个原因。

来看一个例子

@Service
public class MyService {
    public String hello() {
        return "hello";
    }
}

@Slf4j
@RestController
@RequestMapping("/test")
public class MyController {

    @Autowired
    private MyService myService;

    @GetMapping("/public")
    public Object publicHello() {
        return myService.hello();
    }

    @GetMapping("/protected")
    protected Object protectedHello() {
        return myService.hello();
    }

    @GetMapping("/private")
    private Object privateHello() {
        return myService.hello();
    }
}

@EnableAspectJAutoProxy
@SpringBootApplication(exclude = {DataSourceAutoConfiguration.class})
public class MyApplication {
    public static void main(String[] args) {
        SpringApplication.run(MyApplication.class, args);
    }
}

访问 
http://127.0.0.1:8081/test/public 200
http://127.0.0.1:8081/test/protected 200
http://127.0.0.1:8081/test/private 200

如果在这个基础之上再加一个切面：

@Slf4j
@Aspect
@Component
public class MyAspect {

    @Pointcut("execution(* cn.eagle.li.controller..*.*(..))")
    public void controllerSayings() {
    }

    @Before("controllerSayings()")
    public void sayHello() {
        log.info("注解类型前置通知");
    }
}

访问 
http://127.0.0.1:8081/test/public 200
http://127.0.0.1:8081/test/protected 200
http://127.0.0.1:8081/test/private 500：报空指针异常，原因是myService为null的

原因

• public 方法

  

• protected 方法

  

• private 方法

  

大致可以看到原因，public方法和protected方法访问的时候，它的类都是真实的类

而private方法是代理的类

cglib代理的锅

Spring Boot 2.0 开始，默认使用的是cglib代理

@Configuration
@ConditionalOnClass({ EnableAspectJAutoProxy.class, Aspect.class, Advice.class,
		AnnotatedElement.class })
@ConditionalOnProperty(prefix = "spring.aop", name = "auto", havingValue = "true",
		matchIfMissing = true)
public class AopAutoConfiguration {
	@Configuration
	@EnableAspectJAutoProxy(proxyTargetClass = false)
	@ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class",
			havingValue = "false", matchIfMissing = false)
	public static class JdkDynamicAutoProxyConfiguration {
	}

	@Configuration
	@EnableAspectJAutoProxy(proxyTargetClass = true)
	@ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class",
			havingValue = "true", matchIfMissing = true)
	public static class CglibAutoProxyConfiguration {
	}
}

入口

不管public还是private的方法，都是这样执行的。

生成代理类字节码

    public static void main(String[] args) {
        /** 加上这句代码，可以生成代理类的class文件*/
        System.setProperty(DebuggingClassWriter.DEBUG_LOCATION_PROPERTY, "org/springframework/cglib"); 
        SpringApplication.run(MyApplication.class, args);
    }

部分代理类字节码如下：

    protected final Object protectedHello() {
        try {
            MethodInterceptor var10000 = this.CGLIB$CALLBACK_0;
            if (var10000 == null) {
                CGLIB$BIND_CALLBACKS(this);
                var10000 = this.CGLIB$CALLBACK_0;
            }

            return var10000 != null ? var10000.intercept(this, CGLIB$protectedHello$1$Method, CGLIB$emptyArgs, CGLIB$protectedHello$1$Proxy) : super.protectedHello();
        } catch (Error | RuntimeException var1) {
            throw var1;
        } catch (Throwable var2) {
            throw new UndeclaredThrowableException(var2);
        }
    }

   public final Object publicHello() {
        try {
            MethodInterceptor var10000 = this.CGLIB$CALLBACK_0;
            if (var10000 == null) {
                CGLIB$BIND_CALLBACKS(this);
                var10000 = this.CGLIB$CALLBACK_0;
            }

            return var10000 != null ? var10000.intercept(this, CGLIB$publicHello$0$Method, CGLIB$emptyArgs, CGLIB$publicHello$0$Proxy) : super.publicHello();
        } catch (Error | RuntimeException var1) {
            throw var1;
        } catch (Throwable var2) {
            throw new UndeclaredThrowableException(var2);
        }
    }

public和protected方法会生成上述的方法，而private方法是不会生成这样的方法

	private static class DynamicAdvisedInterceptor implements MethodInterceptor, Serializable {
	        @Override
		@Nullable
		public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
			Object oldProxy = null;
			boolean setProxyContext = false;
			Object target = null;
			TargetSource targetSource = this.advised.getTargetSource();
			try {
				if (this.advised.exposeProxy) {
					// Make invocation available if necessary.
					oldProxy = AopContext.setCurrentProxy(proxy);
					setProxyContext = true;
				}
				// Get as late as possible to minimize the time we "own" the target, in case it comes from a pool...
				target = targetSource.getTarget();
				Class<?> targetClass = (target != null ? target.getClass() : null);
				List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
				Object retVal;
				// Check whether we only have one InvokerInterceptor: that is,
				// no real advice, but just reflective invocation of the target.
				if (chain.isEmpty() && Modifier.isPublic(method.getModifiers())) {
					// We can skip creating a MethodInvocation: just invoke the target directly.
					// Note that the final invoker must be an InvokerInterceptor, so we know
					// it does nothing but a reflective operation on the target, and no hot
					// swapping or fancy proxying.
					Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
					retVal = methodProxy.invoke(target, argsToUse);
				}
				else {
					// We need to create a method invocation...
					retVal = new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy).proceed();
				}
				retVal = processReturnType(proxy, target, method, retVal);
				return retVal;
			}
			finally {
				if (target != null && !targetSource.isStatic()) {
					targetSource.releaseTarget(target);
				}
				if (setProxyContext) {
					// Restore old proxy.
					AopContext.setCurrentProxy(oldProxy);
				}
			}
		}
    }

public和protected方法会调用DynamicAdvisedInterceptor.intercept方法，这里面的this.advised.getTargetSource()可以获得真实的目标类，这个目标类是注入成功。

换成JDK动态代理呢

增加配置：

spring:
  aop:
    proxy-target-class: false

增加接口：

@RestController
public interface MyControllerInterface {
    @RequestMapping("/hello/public")
    Object publicHello();

    @RequestMapping("/hello/default")
    default Object defaultHello() {
        return "hi default";
    }
}

@Slf4j
@RestController
@RequestMapping("/test")
public class MyController implements MyControllerInterface {

    @Autowired
    public MyService myService;

    @Override
    @GetMapping("/public")
    public Object publicHello() {
        return myService.hello();
    }

    @GetMapping("/protected")
    protected Object protectedHello() {
        return myService.hello();
    }

    @GetMapping("/private")
    private Object privateHello() {
        return myService.hello();
    }
}

MyControllerInterface头上加@RestController的原因是：

	protected boolean isHandler(Class<?> beanType) {
		return (AnnotatedElementUtils.hasAnnotation(beanType, Controller.class) ||
				AnnotatedElementUtils.hasAnnotation(beanType, RequestMapping.class));
	}

http://127.0.0.1:8081/test/public 404
http://127.0.0.1:8081/test/protected 404
http://127.0.0.1:8081/test/private 404

http://127.0.0.1:8081/hello/public 200
http://127.0.0.1:8081/hello/default 200