为什么不在多模块项目中插入 Maven 属性 ${appengine.app.version}？

【问题标题】：Why doesn't maven property ${appengine.app.version} get interpolated in multi-module project?为什么不在多模块项目中插入 Maven 属性 ${appengine.app.version}？
【发布时间】：2015-02-13 01:22:29
【问题描述】：

我正在尝试使用 Maven 构建多模块 Google App Engine 项目。我有两个模块，一切都很好。但是当我进入管理控制台查看模块时（即在运行mvn appengine:devserver 时转到http://localhost:8080/_ah/admin/modules），我得到了下面的堆栈跟踪：

[INFO] WARNING: /_ah/admin/modules
[INFO] java.lang.NullPointerException: null value in entry: version=null
[INFO]  at com.google.appengine.repackaged.com.google.common.collect.CollectPreconditions.checkEntryNotNull(CollectPreconditions.java:33)
[INFO]  at com.google.appengine.repackaged.com.google.common.collect.ImmutableMapEntry.<init>(ImmutableMapEntry.java:47)
[INFO]  at com.google.appengine.repackaged.com.google.common.collect.ImmutableMapEntry$TerminalEntry.<init>(ImmutableMapEntry.java:67)
[INFO]  at com.google.appengine.repackaged.com.google.common.collect.ImmutableMap.entryOf(ImmutableMap.java:134)
[INFO]  at com.google.appengine.repackaged.com.google.common.collect.ImmutableMap$Builder.put(ImmutableMap.java:204)
[INFO]  at com.google.apphosting.utils.servlet.ModulesServlet$1.apply(ModulesServlet.java:80)
[INFO]  at com.google.apphosting.utils.servlet.ModulesServlet$1.apply(ModulesServlet.java:72)
[INFO]  at com.google.appengine.repackaged.com.google.common.collect.Iterators$9.transform(Iterators.java:841)
[INFO]  at com.google.appengine.repackaged.com.google.common.collect.TransformedIterator.next(TransformedIterator.java:48)
[INFO]  at com.google.appengine.repackaged.com.google.common.collect.ImmutableList.copyOf(ImmutableList.java:271)
[INFO]  at com.google.appengine.repackaged.com.google.common.collect.ImmutableList.copyOf(ImmutableList.java:228)
[INFO]  at com.google.apphosting.utils.servlet.ModulesServlet.doGet(ModulesServlet.java:92)
[INFO]  at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
[INFO]  at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
[INFO]  at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
[INFO]  at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166)
[INFO]  at com.google.appengine.api.socket.dev.DevSocketFilter.doFilter(DevSocketFilter.java:74)
[INFO]  at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
[INFO]  at com.google.appengine.tools.development.ResponseRewriterFilter.doFilter(ResponseRewriterFilter.java:127)
[INFO]  at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
[INFO]  at com.google.appengine.tools.development.HeaderVerificationFilter.doFilter(HeaderVerificationFilter.java:34)
[INFO]  at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
[INFO]  at com.google.appengine.api.blobstore.dev.ServeBlobFilter.doFilter(ServeBlobFilter.java:63)
[INFO]  at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
[INFO]  at com.google.apphosting.utils.servlet.TransactionCleanupFilter.doFilter(TransactionCleanupFilter.java:43)
[INFO]  at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
[INFO]  at com.google.appengine.tools.development.StaticFileFilter.doFilter(StaticFileFilter.java:125)
[INFO]  at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
[INFO]  at com.google.appengine.tools.development.DevAppServerModulesFilter.doDirectRequest(DevAppServerModulesFilter.java:366)
[INFO]  at com.google.appengine.tools.development.DevAppServerModulesFilter.doDirectModuleRequest(DevAppServerModulesFilter.java:349)
[INFO]  at com.google.appengine.tools.development.DevAppServerModulesFilter.doFilter(DevAppServerModulesFilter.java:116)
[INFO]  at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
[INFO]  at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
[INFO]  at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
[INFO]  at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
[INFO]  at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
[INFO]  at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418)
[INFO]  at com.google.appengine.tools.development.DevAppEngineWebAppContext.handle(DevAppEngineWebAppContext.java:98)
[INFO]  at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
[INFO]  at com.google.appengine.tools.development.JettyContainerService$ApiProxyHandler.handle(JettyContainerService.java:491)
[INFO]  at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
[INFO]  at org.mortbay.jetty.Server.handle(Server.java:326)
[INFO]  at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
[INFO]  at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:923)
[INFO]  at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:547)
[INFO]  at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:212)
[INFO]  at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
[INFO]  at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:409)
[INFO]  at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582)
[INFO]

为了构建我的项目，这是我迄今为止所做的：

  1. Setup Skeleton Project with Maven

这会产生一个带有 pom.xml 的单个模块（默认模块），如下所示：

<?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>
    <packaging>war</packaging>
    <version>1.0-SNAPSHOT</version>

    <groupId>mynetbaseapiapp</groupId>
    <artifactId>mynetbaseapiapp</artifactId>

    <properties>
        <appengine.app.version>1</appengine.app.version>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>

    <prerequisites>
        <maven>3.1.0</maven>
    </prerequisites>

    <dependencies>
        <!-- Compile/runtime dependencies -->
        <dependency>
            <groupId>com.google.appengine</groupId>
            <artifactId>appengine-api-1.0-sdk</artifactId>
            <version>1.9.14</version>
        </dependency>
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>servlet-api</artifactId>
            <version>2.5</version>
            <scope>provided</scope>
        </dependency>
        <dependency>
            <groupId>jstl</groupId>
            <artifactId>jstl</artifactId>
            <version>1.2</version>
        </dependency>

        <!-- Test Dependencies -->
        <dependency>
            <groupId>com.google.appengine</groupId>
            <artifactId>appengine-testing</artifactId>
            <version>1.9.14</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>com.google.appengine</groupId>
            <artifactId>appengine-api-stubs</artifactId>
            <version>1.9.14</version>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <!-- for hot reload of the web application-->
        <outputDirectory>${project.build.directory}/${project.build.finalName}/WEB-INF/classes</outputDirectory>
        <plugins>
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>versions-maven-plugin</artifactId>
                <version>2.1</version>
                <executions>
                    <execution>
                        <phase>compile</phase>
                        <goals>
                            <goal>display-dependency-updates</goal>
                            <goal>display-plugin-updates</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <version>3.1</version>
                <artifactId>maven-compiler-plugin</artifactId>
                <configuration>
                    <source>1.7</source>
                    <target>1.7</target>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-war-plugin</artifactId>
                <version>2.4</version>
                <configuration>
                    <archiveClasses>true</archiveClasses>
                    <webResources>
                        <!-- in order to interpolate version from pom into appengine-web.xml -->
                        <resource>
                            <directory>${basedir}/src/main/webapp/WEB-INF</directory>
                            <filtering>true</filtering>
                            <targetPath>WEB-INF</targetPath>
                        </resource>
                    </webResources>
                </configuration>
            </plugin>

            <plugin>
                <groupId>com.google.appengine</groupId>
                <artifactId>appengine-maven-plugin</artifactId>
                <version>1.9.14</version>
                <configuration>
                    <enableJarClasses>false</enableJarClasses>
                    <!-- Comment in the below snippet to bind to all IPs instead of just localhost -->
                    <!-- address>0.0.0.0</address>
                    <port>8080</port -->
                    <!-- Comment in the below snippet to enable local debugging with a remove debugger
                         like those included with Eclipse or IntelliJ -->
                    <!-- jvmFlags>
                      <jvmFlag>-agentlib:jdwp=transport=dt_socket,address=8000,server=y,suspend=n</jvmFlag>
                    </jvmFlags -->
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

  1. 转换为具有多个模块的 EAR 项目布局：

    mkdir mynetbaseapiapp-frontend

    mkdir mynetbaseapiapp-backend

    cp -R src mynetbaseapiapp-frontend/

    cp -R src mynetbaseapiapp-backend/

    rm -rf src

    mkdir mynetbaseapiapp-ear

  2. 通过在 https://github.com/GoogleCloudPlatform/appengine-modules-sample-java 具有类似 2 模块结构的项目之后对它们进行密切建模来配置 Maven pom.xml 文件。

  3. 通过关注https://cloud.google.com/appengine/docs/java/modules/#Java_Configuration手动配置以下文件：

    application.xml - 我必须从头开始创建它，因为 Maven 原型没有提供它

    appengine-application.xml - 我不得不手动创建它

    appengine-web.xml - 我不必创建它，我是从 Maven 原型中获得的：

    mynetbaseapiapp ${appengine.app.version} mynetbaseapiapp-前端
    真的

    <system-properties>
    <property name="java.util.logging.config.file" value="WEB-INF/logging.properties"/>
</system-properties>

我注意到&lt;version&gt;${appengine.app.version}&lt;/version&gt; 行没有得到由Maven 插值的${appengine.app.version} 变量。插值发生在原型生成的原始项目中。但它在我的多模块配置中不起作用。

我哪里出错了？

【问题讨论】：

    标签： java google-app-engine maven


    【解决方案1】：

    版本号${appengine.app.version} 没有得到插值，因为其中一个插件在 pom.xml 中配置不正确。这就是导致java.lang.NullPointerException: null value in entry: version=null 的原因。

    原始 pom.xml 使用 &lt;configuration&gt; 元素处理插值：

      <plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-war-plugin</artifactId>
    <version>2.3</version>
    <configuration>
        <archiveClasses>true</archiveClasses>
        <webResources>
            <!-- in order to interpolate version from pom into appengine-web.xml -->
            <resource>
                <directory>${basedir}/src/main/webapp/WEB-INF</directory>
                <filtering>true</filtering>
                <targetPath>WEB-INF</targetPath>
            </resource>
        </webResources>
    </configuration>        
  </plugin>

    我从https://github.com/GoogleCloudPlatform/appengine-modules-sample-java/blob/master/appengine-modules-guestbook/pom.xml 获得的 pom.xml 缺少 &lt;configuration&gt; 元素：

      <plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-war-plugin</artifactId>
    <version>2.3</version>
  </plugin>

    所以我把它改成了这样：

      <plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-war-plugin</artifactId>
    <version>2.3</version>
    <configuration>
        <archiveClasses>true</archiveClasses>
        <webResources>
            <!-- in order to interpolate version from pom into appengine-web.xml -->
            <resource>
                <directory>${basedir}/src/main/webapp/WEB-INF</directory>
                <filtering>true</filtering>
                <targetPath>WEB-INF</targetPath>
            </resource>
        </webResources>
    </configuration>  
  </plugin>

    这解决了问题，堆栈跟踪消失了，我能够在开发服务器的管理控制台中看到模块。

    【讨论】：

    • 在我找到上面的解决方案后，我发现了一些奇怪的东西。当我将 Maven 项目导入 Eclipse 时，不知何故，src 下的 appengine-web.xml 文件的 元素自动从 ${appengine.app.version} 更改为 1。我重复一遍：appengine-web 的 src 版本。 xml（不是目标版本）通过将 Maven 项目导入 Eclipse 自动执行 ${appengine.app.version} 到 1 的插值。这看起来很奇怪。
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