Maven Dependencies

02 - 固化的Maven依赖

之前已经使用的org.springframework.boot:spring-boot-starter-web和org.springframework.boot:spring-boot-loader均继承自org.springframework.boot:spring-boot-starter-parent，比如版本信息，这些是Maven依赖管理的范畴，降低了Spring Boot依赖管理的成本。

而配置org.springframework.boot:spring-boot-starter-parent也存在一定的限制，因为是单继承方式，限制了其依赖仅是Spring Boot相关。若要需要固化其他类型的依赖就比较麻烦，或者应用的pom.xml有自定义的parent。

在官方文档 中，介绍了相关内容：

Using Spring Boot without the Parent POM
There may be reasons for you not to inherit from the spring-boot-starter-parent POM. You may have your own corporate standard parent that you need to use or you may prefer to explicitly declare all your Maven configuration.

If you do not want to use the spring-boot-starter-parent, you can still keep the benefit of the dependency management (but not the plugin management) by using an import scoped dependency, as follows:

<dependencyManagement>
   <dependencies>
       <dependency>
           <!-- Import dependency management from Spring Boot -->
           <groupId>org.springframework.boot</groupId>
           <artifactId>spring-boot-dependencies</artifactId>
           <version>2.3.2.RELEASE</version>
           <type>pom</type>
           <scope>import</scope>
       </dependency>
   </dependencies>
</dependencyManagement>

对示例项目作出调整，修改pom.xml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">
    <modelVersion>4.0.0</modelVersion>
    <groupId>deep-in-spring-boot</groupId>
    <artifactId>first-spring-boot-application</artifactId>
    <packaging>war</packaging>
    <version>1.0.0-SNAPSHOT</version>
    <name>first-spring-boot-application</name>

<!--    <parent>-->
<!--        <groupId>org.springframework.boot</groupId>-->
<!--        <artifactId>spring-boot-starter-parent</artifactId>-->
<!--        <version>2.3.2.RELEASE</version>-->
<!--    </parent>-->


    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
            <version>2.3.2.RELEASE</version>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>3.8.1</version>
            <scope>test</scope>
        </dependency>
        <!-- spring-boot-loader 分析 -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-loader</artifactId>
            <scope>provided</scope>
        </dependency>
    </dependencies>

    <dependencyManagement>
        <dependencies>
            <dependency>
                <!-- Import dependency management from Spring Boot -->
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-dependencies</artifactId>
                <version>2.3.2.RELEASE</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
        </dependencies>
    </dependencyManagement>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>

去掉<parent>元素，添加了spring-boot-dependencies，之后重新打包：

$ mvn clean package
[INFO] Scanning for projects...
[INFO] 
[INFO] ---------< deep-in-spring-boot:first-spring-boot-application >----------
[INFO] Building first-spring-boot-application 1.0.0-SNAPSHOT
[INFO] --------------------------------[ war ]---------------------------------
[INFO] 
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ first-spring-boot-application ---
[INFO] Deleting /Users/nanlei/Dev/Workspace/first-spring-boot-application/target
[INFO] 
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ first-spring-boot-application ---
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory /Users/nanlei/Dev/Workspace/first-spring-boot-application/src/main/resources
[INFO] 
[INFO] --- maven-compiler-plugin:3.1:compile (default-compile) @ first-spring-boot-application ---
[INFO] Changes detected - recompiling the module!
[WARNING] File encoding has not been set, using platform encoding UTF-8, i.e. build is platform dependent!
[INFO] Compiling 1 source file to /Users/nanlei/Dev/Workspace/first-spring-boot-application/target/classes
[INFO] 
[INFO] --- maven-resources-plugin:2.6:testResources (default-testResources) @ first-spring-boot-application ---
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory /Users/nanlei/Dev/Workspace/first-spring-boot-application/src/test/resources
[INFO] 
[INFO] --- maven-compiler-plugin:3.1:testCompile (default-testCompile) @ first-spring-boot-application ---
[INFO] Changes detected - recompiling the module!
[WARNING] File encoding has not been set, using platform encoding UTF-8, i.e. build is platform dependent!
[INFO] Compiling 1 source file to /Users/nanlei/Dev/Workspace/first-spring-boot-application/target/test-classes
[INFO] 
[INFO] --- maven-surefire-plugin:2.12.4:test (default-test) @ first-spring-boot-application ---
[INFO] Surefire report directory: /Users/nanlei/Dev/Workspace/first-spring-boot-application/target/surefire-reports

-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running deep.in.spring.boot.AppTest
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.004 sec

Results :

Tests run: 1, Failures: 0, Errors: 0, Skipped: 0

[INFO] 
[INFO] --- maven-war-plugin:2.2:war (default-war) @ first-spring-boot-application ---
[INFO] Packaging webapp
[INFO] Assembling webapp [first-spring-boot-application] in [/Users/nanlei/Dev/Workspace/first-spring-boot-application/target/first-spring-boot-application-1.0.0-SNAPSHOT]
[INFO] Processing war project
[INFO] Webapp assembled in [133 msecs]
[INFO] Building war: /Users/nanlei/Dev/Workspace/first-spring-boot-application/target/first-spring-boot-application-1.0.0-SNAPSHOT.war
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  1.686 s
[INFO] Finished at: 2020-08-04T11:11:26+08:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-war-plugin:2.2:war (default-war) on project first-spring-boot-application: Error assembling WAR: webxml attribute is required (or pre-existing WEB-INF/web.xml if executing in update mode) -> [Help 1]
[ERROR] 
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR] 
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException

打包失败了，错误原因是web应用的部署描述文件WEB-INF/web.xml在项目中并不存在，当设置pom.xml的<packaging>war</packaging>时，Maven会执行maven-war-plugin:2.2，WEB-INF/web.xml就是必须的。但是，在作出调整之前却没有这个问题，那么问题就在spring-boot-dependencies和spring-boot-starter-parent的差别之中。

查看spring-boot-starter-parent-2.3.2.RELEASE.pom

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <modelVersion>4.0.0</modelVersion>
  <parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-dependencies</artifactId>
    <version>2.3.2.RELEASE</version>
  </parent>
  <artifactId>spring-boot-starter-parent</artifactId>
  <packaging>pom</packaging>
  <name>spring-boot-starter-parent</name>
  <description>Parent pom providing dependency and plugin management for applications built with Maven</description>
  ...
</project>

可以看出spring-boot-dependencies是spring-boot-starter-parent的parent，也就是说可以将spring-boot-dependencies直接作为parent。

查看spring-boot-dependencies-2.3.2.RELEASE.pom，可以看到

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <modelVersion>4.0.0</modelVersion>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-dependencies</artifactId>
  <version>2.3.2.RELEASE</version>
  <packaging>pom</packaging>
  <name>spring-boot-dependencies</name>
  <description>Spring Boot Dependencies</description>
  ...
  <properties>
    ...
    <spring-boot.version>2.3.2.RELEASE</spring-boot.version>
    ...
    <maven-war-plugin.version>3.2.3</maven-war-plugin.version>
    ...
  </properties>
  <dependencyManagement>
      ...
      <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
        <version>${spring-boot.version}</version>
      </dependency>
      ...
  </dependencyManagement>
  <build>
    <pluginManagement>
      <plugins>
        ...
        <plugin>
          <groupId>org.apache.maven.plugins</groupId>
          <artifactId>maven-war-plugin</artifactId>
          <version>${maven-war-plugin.version}</version>
        </plugin>
        ...
      </plugins>
    </pluginManagement>
  </build>
</project>

无论是Spring Boot的依赖spring-boot-starter-web等，还是插件maven-war-plugin等，均定义在此处。当项目pom.xml文件用<parent>方式引入spring-boot-starter-parent时，mvn package将使用maven-war-plugin:3.2.3和spring-boot-maven-plugin:2.3.2.RELEASE。

相反，<dependencyManagement>方式导入spring-boot-dependencies尽管和前者相同，但此时仅关注<dependencyManagement>，所以maven-war-plugin采用的版本是2.2，手动添加maven-war-plugin:3.2.3到项目pom.xml中：

<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">
    ...
    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
                <version>2.3.2.RELEASE</version>
            </plugin>
            <!-- 保持和spring-boot-dependencies版本一致 -->
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-war-plugin</artifactId>
                <version>3.2.3</version>
            </plugin>
        </plugins>
    </build>
</project>

再次执行打包：

$ mvn clean package
[INFO] Scanning for projects...
[INFO] 
[INFO] ---------< deep-in-spring-boot:first-spring-boot-application >----------
[INFO] Building first-spring-boot-application 1.0.0-SNAPSHOT
[INFO] --------------------------------[ war ]---------------------------------
[INFO] 
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ first-spring-boot-application ---
[INFO] Deleting /Users/nanlei/Dev/Workspace/first-spring-boot-application/target
[INFO] 
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ first-spring-boot-application ---
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory /Users/nanlei/Dev/Workspace/first-spring-boot-application/src/main/resources
[INFO] 
[INFO] --- maven-compiler-plugin:3.1:compile (default-compile) @ first-spring-boot-application ---
[INFO] Changes detected - recompiling the module!
[WARNING] File encoding has not been set, using platform encoding UTF-8, i.e. build is platform dependent!
[INFO] Compiling 1 source file to /Users/nanlei/Dev/Workspace/first-spring-boot-application/target/classes
[INFO] 
[INFO] --- maven-resources-plugin:2.6:testResources (default-testResources) @ first-spring-boot-application ---
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory /Users/nanlei/Dev/Workspace/first-spring-boot-application/src/test/resources
[INFO] 
[INFO] --- maven-compiler-plugin:3.1:testCompile (default-testCompile) @ first-spring-boot-application ---
[INFO] Changes detected - recompiling the module!
[WARNING] File encoding has not been set, using platform encoding UTF-8, i.e. build is platform dependent!
[INFO] Compiling 1 source file to /Users/nanlei/Dev/Workspace/first-spring-boot-application/target/test-classes
[INFO] 
[INFO] --- maven-surefire-plugin:2.12.4:test (default-test) @ first-spring-boot-application ---
[INFO] Surefire report directory: /Users/nanlei/Dev/Workspace/first-spring-boot-application/target/surefire-reports

-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running deep.in.spring.boot.AppTest
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.008 sec

Results :

Tests run: 1, Failures: 0, Errors: 0, Skipped: 0

[INFO] 
[INFO] --- maven-war-plugin:3.2.3:war (default-war) @ first-spring-boot-application ---
[INFO] Packaging webapp
[INFO] Assembling webapp [first-spring-boot-application] in [/Users/nanlei/Dev/Workspace/first-spring-boot-application/target/first-spring-boot-application-1.0.0-SNAPSHOT]
[INFO] Processing war project
[INFO] Webapp assembled in [110 msecs]
[INFO] Building war: /Users/nanlei/Dev/Workspace/first-spring-boot-application/target/first-spring-boot-application-1.0.0-SNAPSHOT.war
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  2.441 s
[INFO] Finished at: 2020-08-04T13:40:35+08:00
[INFO] ------------------------------------------------------------------------

此时打包可以成功，启动程序：

$ java -jar target/first-spring-boot-application-1.0.0-SNAPSHOT.war 
no main manifest attribute, in target/first-spring-boot-application-1.0.0-SNAPSHOT.war

没有发现MANIFEST.MF，说明spring-boot-maven-plugin没有执行，官方文档对此有详细说明：

Create an Executable JAR with Maven
The spring-boot-maven-plugin can be used to create an executable “fat” JAR. If you use the spring-boot-starter-parent POM, you can declare the plugin and your jars are repackaged as follows:

<build>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
        </plugin>
    </plugins>
</build>

If you do not use the parent POM, you can still use the plugin. However, you must additionally add an <executions> section, as follows:

<build>
   <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
            <version>2.3.2.RELEASE</version>
            <executions>
                <execution>
                    <goals>
                        <goal>repackage</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

对pring-boot-maven-plugin设置execution的goal为repackage后，再次打包：

$ mvn clean package
[INFO] Scanning for projects...
(省略部分内容...)
[INFO] --- maven-war-plugin:3.2.3:war (default-war) @ first-spring-boot-application ---
[INFO] Packaging webapp
[INFO] Assembling webapp [first-spring-boot-application] in [/Users/nanlei/Dev/Workspace/first-spring-boot-application/target/first-spring-boot-application-1.0.0-SNAPSHOT]
[INFO] Processing war project
[INFO] Webapp assembled in [116 msecs]
[INFO] Building war: /Users/nanlei/Dev/Workspace/first-spring-boot-application/target/first-spring-boot-application-1.0.0-SNAPSHOT.war
[INFO] 
[INFO] --- spring-boot-maven-plugin:2.3.2.RELEASE:repackage (default) @ first-spring-boot-application ---
[INFO] Replacing main artifact with repackaged archive
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  3.028 s
[INFO] Finished at: 2020-08-04T13:56:06+08:00
[INFO] ------------------------------------------------------------------------

Maven日志中显示了spring-boot-maven-plugin:2.3.2.RELEASE插件的执行信息，再次启动程序：

$ java -jar target/first-spring-boot-application-1.0.0-SNAPSHOT.war 

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.3.2.RELEASE)

2020-08-04 13:58:03.282  INFO 4253 --- [           main] deep.in.spring.boot.App                  : Starting App on nanleis-MacBook-Pro.local with PID 4253 (/Users/nanlei/Dev/Workspace/first-spring-boot-application/target/first-spring-boot-application-1.0.0-SNAPSHOT.war started by nanlei in /Users/nanlei/Dev/Workspace/first-spring-boot-application)
2020-08-04 13:58:03.285  INFO 4253 --- [           main] deep.in.spring.boot.App                  : No active profile set, falling back to default profiles: default
2020-08-04 13:58:04.239  INFO 4253 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8080 (http)
2020-08-04 13:58:04.251  INFO 4253 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
2020-08-04 13:58:04.251  INFO 4253 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet engine: [Apache Tomcat/9.0.37]
2020-08-04 13:58:04.682  INFO 4253 --- [           main] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
2020-08-04 13:58:04.682  INFO 4253 --- [           main] w.s.c.ServletWebServerApplicationContext : Root WebApplicationContext: initialization completed in 1342 ms
2020-08-04 13:58:04.843  INFO 4253 --- [           main] o.s.s.concurrent.ThreadPoolTaskExecutor  : Initializing ExecutorService 'applicationTaskExecutor'
2020-08-04 13:58:05.092  INFO 4253 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8080 (http) with context path ''
2020-08-04 13:58:05.105  INFO 4253 --- [           main] deep.in.spring.boot.App                  : Started App in 2.212 seconds (JVM running for 2.633)

至此项目可以正常启动了，在调整依赖配置的过程中(<parent>方式到<dependencyManagement>方式)，遇到了如下问题：

• maven-war-plugin插件在版本上的差异
  • 2.2 版本默认的打包规则是必须存在Web应用部署描述文件WEB-INF/web.xml
  • 3.2.3 版本中则调整了该行为
• spring-boot-maven-plugin插件在使用时注意添加repackage的goal，否则可能不会添加Spring Boot的引导依赖，从而无法启动程序
• 出于习惯，通常不将spring-boot-dependencies直接作为<parent>，尽管spring-boot-starter-parent也只是简单继承了spring-boot-dependencies