如果我使用的库的源和目标级别为1.7，如何将我的项目编译为java 1.5？

Question

I'm working on a web application which needs to be run on a legacy WebSphere server which runs on an 1.5 JVM.

My problem is that if I set the source/target level to 1.5 in my pom file it works fine

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <configuration>
        <source>1.5</source>
        <target>1.5</target>
    </configuration>
</plugin>

until I add a dependency to my project:

<dependency>
    <groupId>com.sparkjava</groupId>
    <artifactId>spark-core</artifactId>
    <version>1.1.1</version>
</dependency>

I checked its pom file and it has 1.7 as source/target:

<configuration>
        <source>1.7</source>
        <target>1.7</target>
        <optimize>true</optimize>
        <debug>true</debug>
</configuration>

I get this error when I try to start up my app:

Exception in thread "main" java.lang.UnsupportedClassVersionError: Bad version number in .class file
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:621)
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:260)
    at java.net.URLClassLoader.access$100(URLClassLoader.java:56)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:195)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:268)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:252)
    at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:320)

If I set the target level in my project maven fails with another error:

[ERROR] javac: source release 1.7 requires target release 1.7

What can I do to solve this problem?

Answer 1

You're using maven, and that means the spark-core dependency is being pulled from maven central as compiled bytecode in a .jar file.

The pom for that project on github has it being compiled to Java 7 bytecode. I'm willing to bet money that's what is uploaded to maven central.

The only way it would be possible for you to target Java 5 and use that library would be to clone the project, modify its pom to specify 1.5, and build your own jar.

That said, it is highly unlikely that it will build for a ten year old version of Java given that they specify 1.7 in the pom; they are probably using Java 1.7 features.

Answer 2

抓取代码，将其编译为1.5（如果它编译）并插入到您自己的存储库中。