Android应用程序项目中的依赖库项目的NoClassDefFoundError（使用maven作为构建工具）

Question

I developed two Android library projects: Project A (MyBaseA) & Project B(MyBaseB) .

I am using maven as build tool, MyBaseA is a <dependency> of MyBaseB .

I also have an Android app project which uses the built MyBaseB.jar as library, this jar is added to the build path of the Android app project.

For library project A , the maven coordinate definition in pom.xml is:

<?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/maven-v4_0_0.xsd">

    <modelVersion>4.0.0</modelVersion>
    <groupId>com.my.base.a</groupId>
    <artifactId>MyBaseA</artifactId>
    <version>1.0</version>
    <packaging>jar</packaging>
    <name>MyBaseA</name>
    ...

For library project B which is dependent on above library project A project, the pom.xml is:

<?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/maven-v4_0_0.xsd">

    <modelVersion>4.0.0</modelVersion>
    <groupId>com.my.base.b</groupId>
    <artifactId>MyBaseB</artifactId>
    <version>1.0</version>
    <packaging>apklib</packaging>
    <name>MyBaseB</name>

    <dependencies>
      <!--include library project A as dependency-->
      <dependency>
            <groupId>com.my.base.a</groupId>
            <artifactId>MyBaseA</artifactId>
            <version>1.0</version>
       </dependency>
      ...
   </dependencies>

I mvn clean install library project A & B which are all successful.

Under target/ directory of project B , there are MyBaseB.ap_ , MyBaseB.apklib & MyBaseB.jar files.

At this point, I created a new Android app project, in which I included the above MyBaseB.jar to the build path of my new Android app project, and used java classes defined in MyBaseB project, which in turns call functions defined in MyBaseA project.

But when I run my new Android app project, I got exception:

java.lang.NoClassDefFoundError: com.my.base.a.ProjectAClass .

My android app can not find the java class defined in project A . Then I checked the content of MyBaseB.jar , all classes of library project A are not included in MyBaseB.jar . Why? I think the MyBaseB.jar should include project A classes, because the pom has included project A as a <dependency> . Why this error happen?

==================== Update =====================

After I changed library project A 's <packaging> section of pom from jar to apklib , and added the <type>apklib</type> in the dependency of project B (MyBaseB) :

  <dependency>
        <groupId>com.my.base.a</groupId>
        <artifactId>MyBaseA</artifactId>
        <version>1.0</version>
        <type>apklib</type>
   </dependency>

now the MyBaseB.jar contains classes of project A . The above problem solved ( I checked the content of MyBaseB.jar , now, it contains all classes of project A , previously, it didn't.

But the new problem is that, when I run my Android app, the java.lang.NoClassDefFoundError occurs whenever a class of project A is instantiated. That's when app reaches :

ProjectAClass projAclass = new ProjectAClass();

I got java.lang.NoClassDefFoundError . Why? Why it complains even now the MyBaseB.jar contains the ProjectAClass.class.

Answer 1

This happens because the dependency is only used to compile your library. Maven will not do anything more in this case and will presume that the library will be deployed along with the required dependencies.

What you want is either:

• Deploy the MyBaseA library to a location available to your library classpath (probably not possible since you are deploying on android, but I haven't really explored the matter enough to be conclusive), or

• Shade the dependencies inside your library.

Shading is done using the maven-shade-plugin , by adding it the pom of your MyBaseB project:

<project>
  ...
  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-shade-plugin</artifactId>
        <version>2.3</version>
        <executions>
          <execution>
            <phase>package</phase>
            <goals>
              <goal>shade</goal>
            </goals>
            <configuration>
              <filters>
                <filter>
                  <!-- Exclude cryptographic signatures from dependencies, 
                       almost always mandatory (can cause security errors otherwise) -->
                  <artifact>*:*</artifact>
                  <excludes>
                    <exclude>META-INF/*.SF</exclude>
                    <exclude>META-INF/*.DSA</exclude>
                    <exclude>META-INF/*.RSA</exclude>
                  </excludes>
                </filter>
              </filters>
            </configuration>
          </execution>
        </executions>
      </plugin>
    </plugins>
  </build>
  ...
</project>

The plugin will add all dependencies on the compile scope (the default), along with your project, inside a final über-jar.

More detail and examples are available on the maven-shade-plugin website .

Hope this helps.