How Can You Dynamically Modify the Classpath Within a Running Java Process?

Dynamic Classpath Modification within Java Processes

The Java Classpath, which defines the search path for class files and other resources, can be altered dynamically for specific Java processes. This capability is particularly useful when working with environments such as Clojure REPL, where it becomes necessary to add additional jars to the classpath without restarting the process.

While it is possible to modify the system classpath, this approach is not portable and lacks guarantees. Instead, it is recommended to define a new ClassLoader, which adheres to the hierarchical structure of ClassLoaders in Java. Specifically, any class that refers to a class X must be loaded in the same or a child ClassLoader as X.

One approach is to create a URLClassLoader with URLs that are not present in the URLs of the parent ClassLoader. This can be done using the following code:

<code class="java">URL[] url={new URL("file://foo")};
URLClassLoader loader = new URLClassLoader(url);</code>

Another approach involves modifying the current thread's ClassLoader and using reflection to add the desired URL to the system ClassLoader (assuming it is a URLClassLoader):

<code class="java">ClassLoader currentThreadClassLoader = Thread.currentThread().getContextClassLoader();
URLClassLoader urlClassLoader = new URLClassLoader(new URL[] { new File("mtFile").toURL() }, currentThreadClassLoader);
Thread.currentThread().setContextClassLoader(urlClassLoader);</code>

Finally, it is important to note that Java 9 and newer require the use of the Instrumentation API with a Java Agent to add jar files to the classpath. This can be achieved by adding the Launcher-Agent-Class attribute to the manifest of the jar file to start an embedded Agent.

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