How to know if a jar is self contained or needs external resources?

Question

I have a jar file that is giving errors that have led to this question. How do I know if a jar is self contained or if it is loading external resources?

The jar references a license file (*.lic) in the main constructor. Shouldn't this be included in the final jar build?

Here is the main class:

public static void main(String[] args) {
    checkLicense();
}

private static void checkLicense() {
    InputStream licenseStream = null;

    try {
        licenseStream = Main.class.getClassLoader().getResourceAsStream("Vendor.lic");
        if (licenseStream == null) {
            throw new NullPointerException();
        }
    } catch (Exception e) {
        System.err.println("Missing Vendor license resource");
        System.exit(1);
    }

    try {
        License license = new License();
        license.setLicense(licenseStream);
    } catch (Exception e) {
        System.err.println("Invalid Vendor license");
        System.exit(1);
    }

If the Vendor.lic is not there then there is an error, "Missing Vendor license resource".

If the Vendor.lic is there but the classes folder is not there I get another error for "java.lang.ClassNotFoundException".

I see two other .jar files in the same directory.

Can the license be included? Does the jar need other files to work correctly? It seems like it won't work if run independently outside of the folder structure it is in that looks something like this:

/myproject.jar
/classes/generatedsources
/classes/com/mydomain/myproject/Main.class

I create the jar using mvn clean package.

FYI I'd like it to be self contained.

There is a POM.XML and that lists two entries in the dependencies node. There is also a copy dependencies in the plugin\executions node.

Jar Manifest

Manifest-Version: 1.0
Created-By: Maven JAR Plugin 3.3.0
Build-Jdk-Spec: 19
Class-Path: vendor-1.0.0-jdk16.jar helper-library-1.0.0.jar
Specification-Title: myproject
Specification-Version: 1.0
Implementation-Title: myproject
Implementation-Version: 1.0.0
Main-Class: com.domain.project.Main

The pom.xml:

Original:

  <properties>
    <maven.compiler.source>11</maven.compiler.source>
    <maven.compiler.target>11</maven.compiler.target>
  </properties>

Adjusted to relieve the problems errors:

  <properties>
    <maven.compiler.source>12</maven.compiler.source>
    <maven.compiler.target>12</maven.compiler.target>
  </properties>

Problems inspector prior to changing value:

More of the pom.xml:

  <groupId>com.myproject</groupId>
  <artifactId>myproject</artifactId>
  <packaging>jar</packaging>
  <version>1.0.0</version>

  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-jar-plugin</artifactId>
        <configuration>
          <archive>
            <manifest>
              <mainClass>com.mydomain.myproject.Main</mainClass>
              <addClasspath>true</addClasspath>
              <addDefaultImplementationEntries>true</addDefaultImplementationEntries>
              <addDefaultSpecificationEntries>true</addDefaultSpecificationEntries>
            </manifest>
          </archive>
        </configuration>
      </plugin>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-dependency-plugin</artifactId>
        <version>2.10</version>
        <executions>
          <execution>
            <id>copy-dependencies</id>
            <phase>package</phase>
            <goals>
              <goal>copy-dependencies</goal>
            </goals>
            <configuration>
              <outputDirectory>${project.build.directory}</outputDirectory>
              <includeScope>compile</includeScope>
            </configuration>
          </execution>
        </executions>
      </plugin>
    </plugins>
...
  <dependencies>
    <dependency>
      <groupId>com.3rdPartyVendorDomain</groupId>
      <artifactId>3rdPartyVendorDomain-library</artifactId>
      <version>10</version>
      <classifier>jdk16</classifier>
    </dependency>
    <dependency>
      <groupId>com.fasterxml.jackson.core</groupId>
      <artifactId>jackson-core</artifactId>
      <version>2.14.2</version>
    </dependency>
  </dependencies>

The Jar is run from the command line using:

java -jar myproject.jar --parameter1 value --parameter2 value

My Java version:

> java --version
java 12.0.1 2019-04-16
Java(TM) SE Runtime Environment (build 12.0.1+12)
Java HotSpot(TM) 64-Bit Server VM (build 12.0.1+12, mixed mode, sharing)

Answer 1

When the source code checks for an external file, and throws an exception when the external file is missing, this has no correlation with the executable JAR file during runtime with the JRE. The file should still execute fine, but will clearly throw an exception and error out without the *.lic file. According to the JAR manifest file, this *.lic file was not included as a reference file.

If you cannot get the JAR to run at all, that is likely because you are executing with an incompatible JRE version. The vendor is likely creating their own JDK based on version 16 from the looks of the referenced JAR.

The JDK <major>.<minor>.<patch>+<build> version and the JRE <major>.<minor>.<patch>+<build> from the java --version command are usually different because they were developed by different teams at Sun Microsystems and Oracle. The JDK compiles the source code and creates the JAR (version is not always shown in the JAR). The JRE executes the JAR.

The best you can do is look for (or make) a software compatibility matrix when dealing with multiple programming frameworks. With this code, you have versions for the Maven plugin, JDK (from built JAR), JRE (from built JAR--for evidence), CLASSPATH packages (for built JAR), and JRE (your environment--for execution).

Think twice before changing the source code. Tampering with a license file like this is against the law if it's not open source.