Java微服务初体验之Sping Boot

微服务架构

微服务架构近年来受到很多人的推崇，什么是微服务架构？先参考一段定义：

微服务架构（Microservices Architecture）是一种架构风格（Architectural Style）和设计模式，提倡将应用分割成一系列细小的服务，每个服务专注于单一业务功能，运行于独立的进程中，服务之间边界清晰，采用轻量级通信机制（如HTTP/REST）相互沟通、配合来实现完整的应用，满足业务和用户的需求。
引用 - 基于容器云的微服务架构实践

简而言之就是服务轻量级化和模块化，可独立部署。其带来的好处包括：解耦合程度更高，屏蔽底层复杂度；技术选型灵活，可方便其他模块调用；易于部署和扩展等。与NodeJs等其他语言相比，Java相对来说实现微服务较为复杂一些，开发一个Web应用需要经历编码－编译打包－部署到Web容器－启动运行四步，但是开源框架Spring Boot的出现，让Java微服务的实现变得很简单，由于内嵌了Web服务器，无需“部署到Web容器”，三步即可实现一个Web微服务。而且由于Spring Boot可以和Spring社区的其他框架进行集成，对于熟悉Spring的开发者来说很容易上手。下面本文会描述一个简易用户管理微服务的实例，初步体验Spring Boot微服务开发。

开发环境

• JDK 1.8
• Maven 3.0+
• Eclipse或其他IDE
• 本文程序可到github下载

添加依赖

maven的pom.xml配置文件

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
    <!-- inherit defaults from Spring Boot -->
    <parent>
    <groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>1.4.0.RELEASE</version>
	</parent>

    <properties>
        <java.version>1.8</java.version>
    </properties>
    <dependencies>
        <!-- Spring Boot starter POMs -->
        <dependency>
        	<groupId>org.springframework.boot</groupId>
        	<artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <!-- Spring Boot JPA POMs -->
        <dependency>
        	<groupId>org.springframework.boot</groupId>
        	<artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>
        <!-- Spring Boot Test POMs -->
        <dependency>
        	<groupId>org.springframework.boot</groupId>
        	<artifactId>spring-boot-starter-test</artifactId>
        </dependency>
        <!-- H2 POMs -->
        <dependency>
        	<groupId>com.h2database</groupId>
        	<artifactId>h2</artifactId>
        </dependency>
    </dependencies>

    <build>
        <finalName>webapp-springboot-angularjs-seed</finalName>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>  
</project>

这里首先继承了spring-boot-starter-parent的默认配置，然后依次引入

• spring-boot-starter-web 用来构建REST微服务
• spring-boot-starter-data-jpa 利用JPA用来访问数据库
• h2 Spring Boot集成的内存数据库
• spring-boot-starter-test 用来构建单元测试，内含JUnit

程序结构

|____sample
| |____webapp
| | |____ws
| | | |____App.java 启动程序
| | | |____controller
| | | | |____UserController.java 用户管理的RESTful API
| | | |____domain
| | | | |____User.java 用户信息
| | | |____exception
| | | | |____GlobalExceptionHandler.java 异常处理
| | | | |____UserNotFoundException.java 用户不存在的异常
| | | |____repository
| | | | |____UserRepository.java 访问用户数据库的接口
| | | |____rest
| | | | |____RestResultResponse.java Rest请求的状态结果
| | | |____service
| | | | |____UserService.java 用户管理的服务

用SpringApplication实现启动程序

import java.util.Arrays;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;
import com.leonlu.code.sample.webapp.ws.domain.User;
import com.leonlu.code.sample.webapp.ws.service.UserService;

@SpringBootApplication
public class App {
    @Bean
    CommandLineRunner init(UserService userService) {
        // add 5 new users after app are started
        return (evt) -> Arrays.asList("john,alex,mike,mary,jenny".split(","))
                            .forEach(item -> {
                                User user = new User(item, (int)(20 + Math.random() * 10));
                                userService.addUser(user);});
    }

    public static void main(String[] args) {
        SpringApplication.run(App.class, args);
    }
}

• @SpringBootApplication相当于@Configuration,@EnableAutoConfiguration,@ComponentScan三个注解。用于自动完成Spring的配置和Bean的构建。
• main方法中的SpringApplication.run将启动内嵌的Tomcat服务器，默认端口为8080
• CommandLineRunner用于在启动后调用UserService，创建5个新用户

利用RestController构建Restful API

import java.util.Collection;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.http.HttpStatus;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.ResponseStatus;
import org.springframework.web.bind.annotation.RestController;
import com.leonlu.code.sample.webapp.ws.domain.User;
import com.leonlu.code.sample.webapp.ws.rest.RestResultResponse;
import com.leonlu.code.sample.webapp.ws.service.UserService;

@RestController
@RequestMapping("/user")
public class UserController {
    private UserService userService;
    @Autowired
    public UserController(UserService userService) {
        this.userService = userService;
    }

    @RequestMapping(method = RequestMethod.GET)
    public Collection<User> getUsers() {
        return userService.getAllUsers();
    }

    @RequestMapping(method = RequestMethod.POST, consumes = "application/json")
    @ResponseStatus(HttpStatus.CREATED)
    public User addUser(@RequestBody User user) {
        if(user.getName() == null || user.getName().isEmpty()) {
            throw new IllegalArgumentException("Parameter 'name' must not be null or empty");
    	}
        if(user.getAge() == null) {
            throw new IllegalArgumentException("Parameter 'age' must not be null or empty");
    	}
        return userService.addUser(user);
    }

    @RequestMapping(value="/{id}", method = RequestMethod.GET)
    public User getUser(@PathVariable("id") Long id) {
        return userService.getUserById(id);
    }

    @RequestMapping(value="/{id}", method = RequestMethod.PUT, consumes = "application/json")
    public User updateUser(@PathVariable("id") String id, @RequestBody User user) {
    	if(user.getName() == null && user.getAge() == null) {
            throw new IllegalArgumentException("Parameter 'name' and 'age' must not both be null");
    	}
        return userService.addUser(user);
    }

    @RequestMapping(value="/{id}", method = RequestMethod.DELETE)
    public RestResultResponse deleteUser(@PathVariable("id") Long id) {
    	try {
            userService.deleteUser(id);
            return new RestResultResponse(true);
    	} catch(Exception e) {
            return new RestResultResponse(false, e.getMessage());
    	}
    }
}

• UserController的构造方法注入了userService，后者提供了用户管理的基本方法
• 所有方法的默认的Http返回状态是200，addUser方法通过添加@ResponseStatus(HttpStatus.CREATED)注解，将返回状态设置为201。方法中抛出的异常的默认Http返回状态为500，而IllegalArgumentException的状态由于在GlobalExceptionHandler中做了定义，设置为了400 bad request.
• 除deleteUser()外，其余方法的返回结果皆为User对象，在Http结果中Spring Boot会自动转换为Json格式。而deleteUser()的返回结果RestResultResponse，是自定义的操作结果的状态

利用CrudRepository访问数据库

import java.util.Optional;
import org.springframework.data.repository.CrudRepository;
import com.leonlu.code.sample.webapp.ws.domain.User;

public interface UserRepository extends CrudRepository<User, Long> {
    Optional<User> findByName(String name);
}

• CrudRepository提供了save(),findAll(), findOne()等通用的JPA方法，这里自定义了findByName方法，相当于执行"select a from User a where a.username = :username"查询
• 这里仅需要定义UserRepository的接口，Spring Boot会自动定义其实现类

UserService封装用户管理基本功能

import java.util.ArrayList;
import java.util.Collection;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import com.leonlu.code.sample.webapp.ws.domain.User;
import com.leonlu.code.sample.webapp.ws.exception.UserNotFoundException;
import com.leonlu.code.sample.webapp.ws.repository.UserRepository;

@Service
public class UserService {
    private UserRepository userRepository;

    @Autowired
    public UserService(UserRepository userRepository) {
    	this.userRepository = userRepository;
    }

    public Collection<User> getAllUsers() {
    	Iterable<User> userIter = userRepository.findAll();
    	ArrayList<User> userList = new ArrayList<User>();
    	userIter.forEach(item -> {
            userList.add(item);
    	});
    	return userList;
    }

    public User getUserById(Long id) {
    	User user =  userRepository.findOne(id);
    	if(user == null) {
            throw new UserNotFoundException(id);
    	}
    	return user;
    }

    public User getUserByName(String name) {
    	return userRepository.findByName(name)
            .orElseThrow(() -> new UserNotFoundException(name));
    }

    public User addUser(User user) {
    	return userRepository.save(user);
    }

    public User updateUser(User user) {
    	return userRepository.save(user);
    }

    public void deleteUser(Long id) {
    	userRepository.delete(id);
    }
}

• 在getUserById()和getUserByName()中，对于不存在的用户，会抛出UserNotFoundException异常。这是一个自定义异常，其返回状态为HttpStatus.NOT_FOUND，即404:

  import org.springframework.http.HttpStatus;
  import org.springframework.web.bind.annotation.ResponseStatus;
  
  @ResponseStatus(HttpStatus.NOT_FOUND)
  public class UserNotFoundException extends RuntimeException{
      public UserNotFoundException(Long userId) {
          super("could not find user '" + userId + "'.");
      }
      public UserNotFoundException(String userName) {
          super("could not find user '" + userName + "'.");
      }
  }

  示例效果如下：

  $ curl localhost:8080/user/12 -i
  HTTP/1.1 404
  Content-Type: application/json;charset=UTF-8
  Transfer-Encoding: chunked
  Date: Sun, 20 Aug 2016 14:00:41 GMT
  
  {"timestamp":1471788041171,"status":404,"error":"Not Found","exception":"com.leonlu.code.sample.webapp.ws.exception.UserNotFoundException","message":"could not find user '12'.","path":"/user/12"}

利用ControllerAdvice完成异常处理

import java.io.IOException;
import javax.servlet.http.HttpServletResponse;
import org.springframework.http.HttpStatus;
import org.springframework.web.bind.annotation.ControllerAdvice;
import org.springframework.web.bind.annotation.ExceptionHandler;

@ControllerAdvice
public class GlobalExceptionHandler {
    @ExceptionHandler(value = IllegalArgumentException.class)  
    public void handleException(Exception e, HttpServletResponse response) throws IOException{
        response.sendError(HttpStatus.BAD_REQUEST.value());
    }  
}

• 要修改Spring Boot的默认异常返回结果，除了通过对自定义对异常添加@ResponseStatus注解之外（如上文中的UserNotFoundException），开发者可通过@ExceptionHandler注解对某些异常的返回状态和返回结果进行自定义。
• @ControllerAdvice的作用是对全局的Controller进行统一的设置

用Maven打包并运行

• 打包 mvn clean package
• 运行 java -jar JAR_NAME
• 用curl访问localhost:8080/user验证结果

  $ curl -i localhost:8080/user
  HTTP/1.1 200
  Content-Type: application/json;charset=UTF-8
  Transfer-Encoding: chunked
  Date: Sun, 20 Aug 2016 14:32:06 GMT
  
  [{"id":1,"name":"john","age":29},{"id":2,"name":"alex","age":29},{"id":3,"name":"mike","age":27},{"id":4,"name":"mary","age":21},{"id":5,"name":"jenny","age":27}]

总结

基于Spring Boot可快速构建Java微服务，而且官方提供了与Docker，Redis，JMS等多种组件集成的功能，有丰富的文档可供参考，值得大家尝试。

参考

1. Building REST services with Spring
2. Accessing Data with JPA
3. Spring Boot Reference Guide
4. SpringBoot : How to do Exception Handling in Rest Application
5. Post JSON to spring REST webservice

扩展阅读

1. 互联网架构为什么要做服务化？
2. 微服务架构多“微”才合适？