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java – 为什么maven属性${appengine.app.version}不在多模块项目中进行插值？

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我正在尝试使用Maven构建一个多模块的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,这就是我迄今为止所做的：

> Setup Skeleton Project with Maven

这导致单个模块(默认模块)带有如下所示的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">

    <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>

>使用多个模块转换为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
>在https://github.com/GoogleCloudPlatform/appengine-modules-sample-java具有类似双模块结构的项目之后,通过对它们进行紧密建模来配置Maven pom.xml文件.
>按照https://cloud.google.com/appengine/docs/java/modules/#Java_Configuration手动配置以下文件：

application.xml – 我不得不从头开始创建这个,因为Maven
原型没有提供它

appengine-application.xml – 同样,我必须手动创建它

appengine-web.xml – 我没有必要创建它,我从Maven原型得到它：

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

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

我注意到的是< version> ${appengine.app.version}< / version>这一行没有得到Maven插入的${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？

解决方法:

版本${appengine.app.version}号没有得到插值,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：条目中的null值的原因：version = null.

原始的pom.xml使用< configuration>处理插值.元件：

  <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< configuration>元件：

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

所以我改成了这个：

  <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上的管理控制台中看到模块.

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