How to invoke a method in the jar? |

We can use URLClassLoader to load classes from a given path.

      URL myJar = new File("jar/LibraryA-1.0-SNAPSHOT.jar").toURI().toURL();
      URLClassLoader clsLoader = new URLClassLoader(
              new URL[] {myJar},
              this.getClass().getClassLoader()
      );
      Class<?> loadedClass = clsLoader.loadClass("com.sean.liba.Main");
      Method method = loadedClass.getDeclaredMethod("print");
      Object instance = loadedClass.newInstance();
      method.invoke(instance);

      // Output: Hello World!

Let’s look at other use cases. What if you have two jars, and liba.jar deppends on another class in the libb.jar?

Take the above example, the print method has a dependency on the com.sean.libb.Caculator class. If we dont’t change the code, and run it again, you will get an error immediately.

Caused by: java.lang.NoClassDefFoundError: com/sean/lib/Calculator

To fix the issue, we need to include the path of the libb.jar for the URLClassLoader. Following is the example:

      URL myJar = new File("jar/LibraryA-1.0-SNAPSHOT.jar").toURI().toURL();
      // Decalre the URL for the libb.jar
      URL myJar2 = new File("Application/jar/v1/LibraryB-1.0-SNAPSHOT.jar").toURI().toURL();
      URLClassLoader clsLoader = new URLClassLoader(
              new URL[] {myJar, myJar2},
              this.getClass().getClassLoader()
      );
      Class<?> loadedClass = clsLoader.loadClass("com.sean.liba.Main");
      Method method = loadedClass.getDeclaredMethod("print");
      Object instance = loadedClass.newInstance();
      method.invoke(instance);

      // Output: Hello World! and 1 + 1 = 2!

Another interesting question is what if I have another libc.jar depends on the libb.jar. and both liba.jar and libc.jar reference to a singleton class Caculator. How would the singleton Caculator behave? Do they share the same instance?

The answer is it depends on how you use the URLClassLoader. If you load all the jar into the same URLClassLoader, then yes, both liba.jar and libc.jar will share the same intance of the singleton Caculator. But if you load liba.jar and libc.jar on the two difference URLClassLoader, then they will reference to seperate identical instnace of the singleton Caculator.

See the full example.

How to invoke a method in the jar?

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