手写OpenFeign（简易版）

1. 前言

今日有一个需求中需要调用其他服务的接口，由于该项目并非SpringCloud项目，所以先使用OkHttp实现远程调用，但是总觉得有点low，于是想手写一个远程调用组件，当然其实你不自己写，使用OpenFeign也是可以的。

2. 原理说明

项目启动时，扫描带有自定义注解的接口，创建动态代理，并注册到IOC容器。
其实OpenFeign的原理也是如此，只不过OpenFeign中逻辑复杂，但最核心不外乎动态代理、注册BeanDefinition

3. 远程调用组件实现—自定义注解

3.1 添加Spring依赖

新建Module：Remoting，添加以下依赖：

<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-context</artifactId>
</dependency>
<dependency>
    <groupId>org.projectlombok</groupId>
    <artifactId>lombok</artifactId>
</dependency>

3.2 编写@EnableRemoting注解

此注解作用与OpenFeign的@EnableFeignClients作用一致，用于扫描我们自定义@RemoteClient注解。

com.xczs.remoting.register.RemoteClientsRegistrar;
import org.springframework.context.annotation.Import;

import java.lang.annotation.*;

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Import(RemoteClientsRegistrar.class)
public @interface EnableRemoting {

    /**
     * Alias for the {@link #basePackages()} attribute. Allows for more concise annotation
     * declarations e.g.: {@code @ComponentScan("org.my.pkg")} instead of
     * {@code @ComponentScan(basePackages="org.my.pkg")}.
     * @return the array of 'basePackages'.
     */
    String[] value() default {};

    /**
     * Base packages to scan for annotated components.
     * <p>
     * {@link #value()} is an alias for (and mutually exclusive with) this attribute.
     * <p>
     * @return the array of 'basePackages'.
     */
    String[] basePackages() default {};

}

3.3 编写@RemoteClient注解

此注解作用与OpenFeign的@FeignClient作用一致，用于标识我们需要生成代理的接口，创建代理类并注册到Spring的IOC容器。

import java.lang.annotation.*;

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface RemoteClient {

    String value() default "";

    /**
     * @return 绝对 URL 或可解析的主机名（协议是可选的）。
     */
    String url() default "";

}

3.4 编写@GetMapping注解

用于标识该方法是get方法。

import java.lang.annotation.*;

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface GetMapping {

    String value() default "";

}

4. 远程调用组件实现—生成代理类

4.1 编写自定义BeanDefinition注册器

RemoteClientsRegistrar.java

import com.xczs.remoting.annotation.EnableRemoting;
import com.xczs.remoting.scanner.RemoteClientClassPathBeanDefinitionScanner;
import org.springframework.beans.factory.support.BeanDefinitionRegistry;
import org.springframework.context.annotation.ImportBeanDefinitionRegistrar;
import org.springframework.core.annotation.AnnotationAttributes;
import org.springframework.core.type.AnnotationMetadata;

import java.util.Map;
import java.util.Objects;

public class RemoteClientsRegistrar implements ImportBeanDefinitionRegistrar {

    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
        Map<String, Object> attributes = importingClassMetadata.getAnnotationAttributes(EnableRemoting.class.getName());
        AnnotationAttributes annotationAttributes = AnnotationAttributes.fromMap(attributes);
        String[] basePackages = null;
        if (Objects.nonNull(annotationAttributes.getStringArray("value"))) {
            basePackages = annotationAttributes.getStringArray("value");
        }
        if (Objects.isNull(basePackages) || basePackages.length == 0) {
            basePackages = (String[])attributes.get("basePackages");
        }
        if (Objects.isNull(basePackages) || basePackages.length == 0) {
            throw new IllegalArgumentException("@EnableRemoting注解value或basePackages属性不可同时为空.");
        }
        RemoteClientClassPathBeanDefinitionScanner scanner = new RemoteClientClassPathBeanDefinitionScanner(registry, false);
        scanner.doScan(basePackages);
    }
}

4.2 编写自定义包扫描器

RemoteClientClassPathBeanDefinitionScanner.java

import com.xczs.remoting.annotation.RemoteClient;
import com.xczs.remoting.bean.RemoteClientFactoryBean;
import lombok.extern.slf4j.Slf4j;
import org.springframework.beans.factory.annotation.AnnotatedBeanDefinition;
import org.springframework.beans.factory.config.BeanDefinition;
import org.springframework.beans.factory.config.BeanDefinitionHolder;
import org.springframework.beans.factory.support.BeanDefinitionRegistry;
import org.springframework.beans.factory.support.GenericBeanDefinition;
import org.springframework.context.annotation.ClassPathBeanDefinitionScanner;
import org.springframework.core.type.AnnotationMetadata;
import org.springframework.core.type.filter.AnnotationTypeFilter;
import org.springframework.util.Assert;

import java.util.Set;

@Slf4j
public class RemoteClientClassPathBeanDefinitionScanner extends ClassPathBeanDefinitionScanner {

    public RemoteClientClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry, boolean useDefaultFilters) {
        super(registry, useDefaultFilters);
    }

    @Override
    public Set<BeanDefinitionHolder> doScan(String... basePackages) {
        // 添加过滤器, 只扫描添加了 RemoteClient 注解的类
        addIncludeFilter(new AnnotationTypeFilter(RemoteClient.class));
        Set<BeanDefinitionHolder> beanDefinitionHolderSet = super.doScan(basePackages);
        // 对扫描到的数据进行代理处理
        processBeanDefinitions(beanDefinitionHolderSet);
        return beanDefinitionHolderSet;
    }

    @Override
    protected boolean isCandidateComponent(AnnotatedBeanDefinition beanDefinition) {
        return beanDefinition.getMetadata().isInterface() && beanDefinition.getMetadata().isIndependent();
    }

    private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitionHolderSet) {
        beanDefinitionHolderSet.forEach(beanDefinitionHolder -> {
            BeanDefinition beanDefinition = beanDefinitionHolder.getBeanDefinition();
            if (beanDefinition instanceof AnnotatedBeanDefinition) {
                AnnotationMetadata annotationMetadata = ((AnnotatedBeanDefinition) beanDefinition).getMetadata();
                Assert.isTrue(annotationMetadata.isInterface(), "@RemoteClient只能使用在接口上");
            }
            // 设置工厂等操作需要基于GenericBeanDefinition, BeanDefinitionHolder是其子类
            GenericBeanDefinition definition = (GenericBeanDefinition) beanDefinition;
            // 获取接口的全路径名称
            String beanClassName = definition.getBeanClassName();
            // 设置构造函数参数
            definition.getConstructorArgumentValues().addGenericArgumentValue(beanClassName);
            // 设置工厂
            definition.setBeanClass(RemoteClientFactoryBean.class);
            definition.setAutowireMode(GenericBeanDefinition.AUTOWIRE_BY_TYPE);
        });
    }
}

4.3 编写FactoryBean

RemoteClientFactoryBean.java

import org.springframework.beans.factory.FactoryBean;

import java.lang.reflect.Proxy;

public class RemoteClientFactoryBean<T> implements FactoryBean<T> {

    private Class<T> type;

    public RemoteClientFactoryBean(Class<T> type) {
        this.type = type;
    }

    @Override
    public T getObject() throws Exception {
        // 因为 DefaultRemoteClient 需要Class<T>作为参数, 所以该类包含一个Class<T>的成员, 通过构造函数初始化
        return (T) Proxy.newProxyInstance(type.getClassLoader(), new Class[]{type},
                new DefaultRemoteClient<>(type));
    }

    @Override
    public Class<?> getObjectType() {
        // 该方法返回的getObject()方法返回对象的类型,这里是基于type生成的代理对象, 所以类型就是上面定义的type
        return type;
    }
}

4.4 编写远程调用接口的默认实现

DefaultRemoteClient.java

import com.xczs.core.utils.OkHttpUtils;
import com.xczs.remoting.annotation.GetMapping;
import com.xczs.remoting.annotation.RemoteClient;
import lombok.extern.slf4j.Slf4j;
import okhttp3.Response;

import java.lang.annotation.Annotation;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;

@Slf4j
public class DefaultRemoteClient<T> implements InvocationHandler {

    /**
     * 这里声明一个Class, 用来接收接口声明的泛型实际类型的class, T是声明的实体类类型
     */
    private Class<T> type;

    public DefaultRemoteClient(Class<T> type) {
        this.type = type;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        // Object 方法，走原生方法, 比如 hashCode()
        if (Object.class.equals(method.getDeclaringClass())) {
            return method.invoke(this, args);
        }
        // 其它走动态代理
        Class<?> declaringClass = method.getDeclaringClass();
        RemoteClient remoteClient = declaringClass.getAnnotation(RemoteClient.class);
        String domain = remoteClient.url();

        Annotation[] annotations = method.getAnnotations();
        for (Annotation annotation : annotations) {
            if (annotation instanceof GetMapping) {
                GetMapping getAnno = (GetMapping) annotation;
                String uri = getAnno.value();
                String url = domain + uri;
                Response response = OkHttpUtils.doExecuteGet(url);
                String resp = response.body().string();
                return resp;
            }
        }

        return null;
    }
}

5. 成果展现

服务提供方：

远程调用接口：

测试类：

调用结果：