Spring Cloud 启动错误 - java.lang.No

最近想把项目中的一个功能设计成一个microservice，降低系统件的耦合度，方便后期维护与扩展。之前没有接触过microservice，只是知道microservice最近两年比较火。作为技术人员总喜欢折腾新技术，所以决定上网找个quick starter，结果找到了这篇https://spring.io/blog/2015/07/14/microservices-with-spring。

按照文中的步骤，首先需要创建一个服务注册（service registration）项目。文中给出的guide很简洁，自己新建了一个spring boot project，copy了提供的POM配置，结果运行时却碰到了如下错误：

java.lang.NoSuchMethodError: org.springframework.boot.builder.SpringApplicationBuilder.<init>([Ljava/lang/Object;)V
    at org.springframework.cloud.bootstrap.BootstrapApplicationListener.bootstrapServiceContext(BootstrapApplicationListener.java:161)
    at org.springframework.cloud.bootstrap.BootstrapApplicationListener.onApplicationEvent(BootstrapApplicationListener.java:102)
    at org.springframework.cloud.bootstrap.BootstrapApplicationListener.onApplicationEvent(BootstrapApplicationListener.java:68)
    at org.springframework.context.event.SimpleApplicationEventMulticaster.doInvokeListener(SimpleApplicationEventMulticaster.java:172)
    at org.springframework.context.event.SimpleApplicationEventMulticaster.invokeListener(SimpleApplicationEventMulticaster.java:165)
    at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:139)
    at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:127)
    at org.springframework.boot.context.event.EventPublishingRunListener.environmentPrepared(EventPublishingRunListener.java:74)
    at org.springframework.boot.SpringApplicationRunListeners.environmentPrepared(SpringApplicationRunListeners.java:54)
    at org.springframework.boot.SpringApplication.prepareEnvironment(SpringApplication.java:358)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:317)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1255)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1243)
    at demo.ms.robossliu.demo.DemoApplication.main(DemoApplication.java:14)

尤其是第一行信息，似乎提示一个莫名其妙的无此方法错误。Spring boot框架很多组件都是基于依赖注入（DI），有些错误在编译时可能无法被检测出来，只有在运行时才暴露出来。既然提示无此方法，那么肯定是某个组件（包）有问题：要么未被注入；要么版本版本不对。

也没有过多考虑，直接bing一下（虽然想用google，但确实慢），看了几篇stackoverflow上的文章，基本可以断定是依赖包之间存在的兼容性问题，导致bean未能正确被注入。自己检查了这份copy出来的POM文件：

<parent>
       <groupId>org.springframework.boot</groupId>
       <artifactId>spring-boot-starter-parent</artifactId>
       <version>2.0.0.RELEASE</version>
   </parent>

   <dependencies>
       <dependency>
           <!-- Setup Spring Boot -->
           <groupId>org.springframework.boot</groupId>
           <artifactId>spring-boot-starter</artifactId>
       </dependency>

       <dependency>
           <!-- Setup Spring MVC & REST, use Embedded Tomcat -->
           <groupId>org.springframework.boot</groupId>
           <artifactId>spring-boot-starter-web</artifactId>
       </dependency>

       <dependency>
           <!-- Spring Cloud starter -->
           <groupId>org.springframework.cloud</groupId>
           <artifactId>spring-cloud-starter</artifactId>
       </dependency>

       <dependency>
           <!-- Eureka for service registration -->
           <groupId>org.springframework.cloud</groupId>
           <artifactId>spring-cloud-starter-eureka-server</artifactId>
       </dependency>
   </dependencies>

  <!-- Spring Cloud dependencies -->
   <dependencyManagement>
       <dependencies>
           <dependency>
               <groupId>org.springframework.cloud</groupId>
               <artifactId>spring-cloud-dependencies</artifactId>
               <version>Edgware.SR3</version>
               <type>pom</type>
               <scope>import</scope>
           </dependency>
       </dependencies>
   </dependencyManagement>

发现自己使用的是spring boot 2.0.0版本，spring cloud使用是Edgware.SR3版本，难道是两者冲突？对于Edgware.SR3是哪个版本的spring cloud以及依赖的spring boot版本，自己不是很清楚，毕竟之前没有玩过。打开了spring cloud官网，发现Edgware.SR3版本依赖于spring boot 1.5.x。如此看来是这两个组件的版本发生了冲突，所以解决方法也比较简单：要么降级spring boot版本，要么升级spring cloud版本。内心一直秉持着一种理念“新技术总是更好”，于是果断选择升级spring cloud至最新版本Finchley。修改POM文件之后，编译运行一切OK了。

修改之后的POM文件如下，供大家参考一下：

<?xml version="1.0" encoding="UTF-8"?>
<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/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>demo.ms.robossliu</groupId>
    <artifactId>demo</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>jar</packaging>
    <name>demo</name>
    <description>Demo project for Spring Boot</description>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.0.0.RELEASE</version>
        <relativePath/>
    </parent>
    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <java.version>1.8</java.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-configuration-processor</artifactId>
            <version>2.0.0.RELEASE</version>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-eureka-server</artifactId>
            <version>1.4.4.RELEASE</version>
        </dependency>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-config</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-eureka</artifactId>
            <version>1.4.4.RELEASE</version>
        </dependency>
    </dependencies>
    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>org.springframework.cloud</groupId>
                <artifactId>spring-cloud-dependencies</artifactId>
                <version>Finchley.BUILD-SNAPSHOT</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
        </dependencies>
    </dependencyManagement>
    <repositories>
        <repository>
            <id>spring-snapshots</id>
            <name>Spring Snapshots</name>
            <url>https://repo.spring.io/libs-snapshot</url>
            <snapshots>
                <enabled>true</enabled>
            </snapshots>
        </repository>
    </repositories>
    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>