下載代碼切換分支
首先到GitHub上clone下Spring Boot的代碼：
git clone https://github.com/spring-projects/spring-boot.git
由於Spring Boot的發佈版本代碼都在tag上，所以需要使用git tag命令查看所有的tag：
git tag
然後切換到名爲v2.0.0.RELEASE的tag上：
git checkout -b v2.0.0.RELEASE v2.0.0.RELEASE
這樣，代碼就被保存到本地分支v2.0.0.RELEASE上了。
源碼編譯
Spring Boot官方建議使用./mvnw clean install或者標準的mvn clean install命令來編譯源代碼，如果要使用標準的mvn命令的話，Maven的版本要求在3.5.0或以上。導入IDEA源碼視圖如下：
接着切換到spring-boot根目錄下，執行如下命令，我這裏使用的Maven版本是3.5.4：
mvn -Dmaven.test.skip=true clean install
以上命令對Spring Boot源碼打包並安裝到本地maven倉庫，在打包過程中會忽略測試，因爲運行單元測試時間特別長，下載源碼的目的是學習和分析Spring Boot的原理，而並不是做定製開發，因此一些不影響學習的單元測試可以忽略掉，可以不關注單測的結果。命令執行結果如下：
打包失敗主要是因爲失敗單元測試引起的，這些單元測試會影響最終編譯打包結果：
org.springframework.boot.gradle.plugin.OnlyDependencyManagementIntegrationTests > dependencyManagementCanBeConfiguredUsingCoordinatesConstant [Gradle default] FAILED
java.lang.RuntimeException at OnlyDependencyManagementIntegrationTests.java:42
Caused by: org.gradle.testkit.runner.UnexpectedBuildFailure at OnlyDependencyManagementIntegrationTests.java:42
org.springframework.boot.gradle.plugin.OnlyDependencyManagementIntegrationTests > dependencyManagementCanBeConfiguredUsingCoordinatesConstant [Gradle 4.1] FAILED
java.lang.RuntimeException at OnlyDependencyManagementIntegrationTests.java:42
Caused by: org.gradle.testkit.runner.UnexpectedBuildFailure at OnlyDependencyManagementIntegrationTests.java:42
org.springframework.boot.gradle.plugin.OnlyDependencyManagementIntegrationTests > dependencyManagementCanBeConfiguredUsingCoordinatesConstant [Gradle 4.2] FAILED
java.lang.RuntimeException at OnlyDependencyManagementIntegrationTests.java:42
Caused by: org.gradle.testkit.runner.UnexpectedBuildFailure at OnlyDependencyManagementIntegrationTests.java:42
org.springframework.boot.gradle.plugin.OnlyDependencyManagementIntegrationTests > dependencyManagementCanBeConfiguredUsingCoordinatesConstant [Gradle 4.3] FAILED
java.lang.RuntimeException at OnlyDependencyManagementIntegrationTests.java:42
Caused by: org.gradle.testkit.runner.UnexpectedBuildFailure at OnlyDependencyManagementIntegrationTests.java:42
org.springframework.boot.gradle.plugin.OnlyDependencyManagementIntegrationTests > dependencyManagementCanBeConfiguredUsingCoordinatesConstant [Gradle 4.4.1] FAILED
java.lang.RuntimeException at OnlyDependencyManagementIntegrationTests.java:42
Caused by: org.gradle.testkit.runner.UnexpectedBuildFailure at OnlyDependencyManagementIntegrationTests.java:42
org.springframework.boot.gradle.plugin.OnlyDependencyManagementIntegrationTests > dependencyManagementCanBeConfiguredUsingCoordinatesConstant [Gradle 4.5] FAILED
java.lang.RuntimeException at OnlyDependencyManagementIntegrationTests.java:42
Caused by: org.gradle.testkit.runner.UnexpectedBuildFailure at OnlyDependencyManagementIntegrationTests.java:42
這些失敗的單測屬於spring-boot-project/spring-boot-tools下的spring-boot-gradle-plugin項目，一個比較暴力的解決辦法是直接刪掉這個項目下的src/test/java，不運行這個項目的單測，因爲暫時也用不到它。刪除後再執行：
mvn -Dmaven.test.failure.ignore=true -Dmaven.test.skip=true clean install
以上命令中的-Dmaven.test.failure.ignore=true會使Maven忽略掉失敗的單元測試，等待命令執行5-10分鐘，顯示執行成功：
接下來就可以創建測試項目進行源碼調試了。
測試
打包成功之後，在spring-boot/spring-boot-project目錄下創建一個Spring Boot項目測試一下自己編譯的源碼是否可以正常運行，在spring-boot-project上點擊New->Module爲它創建一個名爲spring-boot-example的子module，接着按照標準Spring Boot服務的順序來創建Application、配置文件application.yml和相應的controller，創建完成後，視圖如下：
代碼依次如下：
1、Application類：
@SpringBootApplication
public class Application {
public static void main(String[] args) {
SpringApplication.run(Application.class, args); }
}
2、WebController類：
@Controller
@RequestMapping(value = "/web", produces = MediaType.APPLICATION_JSON_UTF8_VALUE)
@ResponseBody
public class WebController {
@GetMapping(value = "/test")
public List test() {
List list = new ArrayList();
list.add("這是測試");
return list;
}
}
3、application.yml：
server:
port: 6011
spring:
application:
name: spring-boot-example
4、pom.xml：
<?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">
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-parent</artifactId>
<version>${revision}</version>
<relativePath>../spring-boot-parent</relativePath>
</parent>
<modelVersion>4.0.0</modelVersion>
<artifactId>spring-boot-example</artifactId>
<packaging>jar</packaging>
<name>spring-boot-example Maven Webapp</name>
<!-- FIXME change it to the project's website -->
<url>http://www.example.com</url>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<maven.compiler.source>1.8</maven.compiler.source>
<maven.compiler.target>1.8</maven.compiler.target>
</properties>
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-autoconfigure</artifactId>
</dependency>
<!-- Compile -->
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
<!-- Test -->
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-test</artifactId>
<scope>test</scope>
</dependency>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.11</version>
<scope>test</scope>
</dependency>
</dependencies>
<build>
<finalName>spring-boot-example</finalName>
<plugins>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
<configuration>
<executable>true</executable> <!-- Add -->
</configuration>
<executions>
<execution>
<goals>
<goal>repackage</goal>
</goals>
</execution>
</executions>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-surefire-plugin</artifactId>
<configuration>
<useSystemClassLoader>false</useSystemClassLoader>
</configuration>
</plugin>
</plugins>
</build>
</project>
接下來，還需要將spring-boot-project/spring-boot-parent和spring-boot-project/spring-boot-starters兩個項目的pom.xml裏的maven-checkstyle-plugin信息註釋掉，纔可以運行的測試項目，因爲這個插件會對代碼進行檢查，檢查失敗的話，服務運行不起來。
最後，直接運行Application類，輸出運行成功日誌：
. ____ _ __ _ _
/\\ / ___'_ __ _ _(_)_ __ __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
\\/ ___)| |_)| | | | | || (_| | ) ) ) )
' |____| .__|_| |_|_| |_\__, | / / / /
=========|_|==============|___/=/_/_/_/
:: Spring Boot ::
2018-09-04 11:07:37.046 INFO 4576 --- [ main] o.s.boot.example.Application : Starting Application on Armons-MacBook-Pro.local with PID 4576 (/Users/pengwang/Documents/workspace-oxygen/data/spring-boot/spring-boot-project/spring-boot-example/target/classes started by wangpeng in /Users/pengwang/Documents/workspace-oxygen/data/spring-boot)
2018-09-04 11:07:37.079 INFO 4576 --- [ main] o.s.boot.example.Application : No active profile set, falling back to default profiles: default
2018-09-04 11:07:37.248 INFO 4576 --- [ main] ConfigServletWebServerApplicationContext : Refreshing org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext@75c072cb: startup date [Tue Sep 04 11:07:37 CST 2018]; root of context hierarchy
2018-09-04 11:07:40.565 INFO 4576 --- [ main] o.s.b.w.embedded.tomcat.TomcatWebServer : Tomcat initialized with port(s): 6011 (http)
2018-09-04 11:07:40.619 INFO 4576 --- [ main] o.apache.catalina.core.StandardService : Starting service [Tomcat]
2018-09-04 11:07:40.619 INFO 4576 --- [ main] org.apache.catalina.core.StandardEngine : Starting Servlet Engine: Apache Tomcat/8.5.28
2018-09-04 11:07:40.641 INFO 4576 --- [ost-startStop-1] o.a.catalina.core.AprLifecycleListener : The APR based Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: [/Users/pengwang/Library/Java/Extensions:/Library/Java/Extensions:/Network/Library/Java/Extensions:/System/Library/Java/Extensions:/usr/lib/java:.]
2018-09-04 11:07:40.837 INFO 4576 --- [ost-startStop-1] o.a.c.c.C.[Tomcat].[localhost].[/] : Initializing Spring embedded WebApplicationContext
2018-09-04 11:07:40.838 INFO 4576 --- [ost-startStop-1] o.s.web.context.ContextLoader : Root WebApplicationContext: initialization completed in 3594 ms
2018-09-04 11:07:41.179 INFO 4576 --- [ost-startStop-1] o.s.b.w.servlet.ServletRegistrationBean : Servlet dispatcherServlet mapped to [/]
2018-09-04 11:07:41.183 INFO 4576 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean : Mapping filter: 'characterEncodingFilter' to: [/*]
2018-09-04 11:07:41.184 INFO 4576 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean : Mapping filter: 'hiddenHttpMethodFilter' to: [/*]
2018-09-04 11:07:41.184 INFO 4576 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean : Mapping filter: 'httpPutFormContentFilter' to: [/*]
2018-09-04 11:07:41.184 INFO 4576 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean : Mapping filter: 'requestContextFilter' to: [/*]
2018-09-04 11:07:41.617 INFO 4576 --- [ main] s.w.s.m.m.a.RequestMappingHandlerAdapter : Looking for @ControllerAdvice: org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext@75c072cb: startup date [Tue Sep 04 11:07:37 CST 2018]; root of context hierarchy
2018-09-04 11:07:41.731 INFO 4576 --- [ main] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/web/test],methods=[GET],produces=[application/json;charset=UTF-8]}" onto public java.util.List org.springframework.boot.example.WebController.test()
2018-09-04 11:07:41.738 INFO 4576 --- [ main] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/error]}" onto public org.springframework.http.ResponseEntity<java.util.Map<java.lang.String, java.lang.Object>> org.springframework.boot.autoconfigure.web.servlet.error.BasicErrorController.error(javax.servlet.http.HttpServletRequest)
2018-09-04 11:07:41.739 INFO 4576 --- [ main] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/error],produces=[text/html]}" onto public org.springframework.web.servlet.ModelAndView org.springframework.boot.autoconfigure.web.servlet.error.BasicErrorController.errorHtml(javax.servlet.http.HttpServletRequest,javax.servlet.http.HttpServletResponse)
2018-09-04 11:07:41.791 INFO 4576 --- [ main] o.s.w.s.handler.SimpleUrlHandlerMapping : Mapped URL path [/webjars/**] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler]
2018-09-04 11:07:41.792 INFO 4576 --- [ main] o.s.w.s.handler.SimpleUrlHandlerMapping : Mapped URL path [/**] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler]
2018-09-04 11:07:41.849 INFO 4576 --- [ main] o.s.w.s.handler.SimpleUrlHandlerMapping : Mapped URL path [/**/favicon.ico] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler]
2018-09-04 11:07:42.101 INFO 4576 --- [ main] o.s.j.e.a.AnnotationMBeanExporter : Registering beans for JMX exposure on startup
2018-09-04 11:07:42.175 INFO 4576 --- [ main] o.s.b.w.embedded.tomcat.TomcatWebServer : Tomcat started on port(s): 6011 (http) with context path ''
2018-09-04 11:07:42.180 INFO 4576 --- [ main] o.s.boot.example.Application : Started Application in 6.077 seconds (JVM running for 6.955)
打開瀏覽器訪問http://localhost:6011/web/test，頁面顯示：
[
"這是測試"
]
至此，源碼編譯打包運行完畢。
總結
開源框架之所以被很多人使用，一方面是因爲解決了一些常見的痛點問題，一方面是因爲可靠和健壯，保證用開源框架開發出來的服務高可用。提高服務可靠和健壯的一方面就是單元測試，單元測試雖然繁瑣，但讓我們對自己寫的代碼和別人寫的代碼可靠性瞭然於胸。