You can load one version from classpath and the rest of versions using URLClassLoader by passing parent: null and using reflection to instantiate the members.
An alternative is to use a custom class loader, based on an approach described here that may look like:
import java.net.URL;
import java.net.URLClassLoader;
import java.net.URLStreamHandlerFactory;
public class ParentLastClassLoader extends ClassLoader {
private ClassLoader parentClassLoader;
private URLClassLoader noParentClassLoader;
public ParentLastClassLoader(URL[] urls, ClassLoader parent) {
super(parent);
this.noParentClassLoader = new URLClassLoader(urls, null);
}
public ParentLastClassLoader(URL[] urls) {
super(Thread.currentThread().getContextClassLoader());
this.noParentClassLoader = new URLClassLoader(urls, null);
}
public ParentLastClassLoader(URL[] urls, ClassLoader parent, URLStreamHandlerFactory factory) {
super(parent);
this.noParentClassLoader = new URLClassLoader(urls, null, factory);
}
@Override
protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
try {
// resolve using child class loader
return noParentClassLoader.loadClass(name);
} catch (ClassNotFoundException ex) {
// resolve using parent class loader
return super.loadClass(name, resolve);
}
}
}
I created a POC for this in this Git repo. The details are in the README.md.
POC contents
Considering that you have a library with multiple versions (project v1, v2, v3) consisting on following pieces:
We want our library to have this interface:
public interface Core {
String getVersion();
String getDependencyVersion();
}
Which is implemented by:
package sample.multiversion;
import sample.multiversion.deps.CoreDependency;
public class ImportantCore implements Core {
private Utility utility;
private CoreDependency coreDependency;
public ImportantCore() {
utility = new Utility();
coreDependency = new CoreDependency();
}
public String getVersion() {
return utility.getVersion();
}
public String getDependencyVersion() {
return coreDependency.getVersion();
}
}
that is using another class of the same library:
package sample.multiversion;
public class Utility {
public String getVersion() {
return "core-v1";
}
}
Finally the library (projects v1, v2, v3) have a dependency (projects v1dep, v2dep, v3dep) containing:
package sample.multiversion.deps;
public class CoreDependency {
public String getVersion() {
return "core-dep-v1";
}
}
Then we can load all three versions:
- V1 - from file
- V2 - from file
- V3 - from classpath
The code would be:
// multiple versions of the same library to be used at the same time
URL v1 = Paths.get("./../v1/build/libs/v1.jar").toUri().toURL();
URL v2 = Paths.get("./../v2/build/libs/v2.jar").toUri().toURL();
// library dependencies
URL v1Dep = Paths.get("./../v1dep/build/libs/v1dep.jar").toUri().toURL();
URL v2Dep = Paths.get("./../v2dep/build/libs/v2dep.jar").toUri().toURL();
/**
* version 1 and 2 loaders
* - these loaders do not use the root loader - Thread.currentThread().getContextClassLoader()
* - using the root loader new URLClassLoader(new URL[]{v1, v1Dep}, Thread.currentThread().getContextClassLoader());
* will solve any class with the root loader and if no class is found then the child loader will be used
* - because version 3 is loaded from classpath, the root loader should not be used to load version 1 and 2
* - null needs to be passed to parent argument, else will not work
*/
URLClassLoader loaderV1 = new URLClassLoader(new URL[]{v1, v1Dep}, null);
URLClassLoader loaderV2 = new URLClassLoader(new URL[]{v2, v2Dep}, null);
/**
* Use custom class loader for loading classes first from children and last from parent
*/
ParentLastClassLoader loaderV1Alt = new ParentLastClassLoader(new URL[]{v1, v1Dep});
ParentLastClassLoader loaderV2Alt = new ParentLastClassLoader(new URL[]{v2, v2Dep});
ParentLastClassLoader loaderV3Alt = new ParentLastClassLoader(new URL[]{});
// get class from loader
Class<?> coreV1Class = loaderV1.loadClass("sample.multiversion.ImportantCore");
Class<?> coreV2Class = loaderV2.loadClass("sample.multiversion.ImportantCore");
// get class from loader - custom version
Class<?> coreV1AltClass = loaderV1Alt.loadClass("sample.multiversion.ImportantCore");
Class<?> coreV2AltClass = loaderV2Alt.loadClass("sample.multiversion.ImportantCore");
Class<?> coreV3AltClass = loaderV3Alt.loadClass("sample.multiversion.ImportantCore");
// create class instance
Object coreV1Instance = coreV1Class.newInstance();
Object coreV2Instance = coreV2Class.newInstance();
// create class instance - obtained from custom class loader
Object coreV1AltInstance = coreV1AltClass.newInstance();
Object coreV2AltInstance = coreV2AltClass.newInstance();
// note that this is loaded from classpath
Core coreV3Instance = new ImportantCore();
Core coreV3AltInstance = (Core)coreV3AltClass.newInstance();
// get version
String v1Str = (String) coreV1Class.getMethod("getVersion").invoke(coreV1Instance);
String v2Str = (String) coreV2Class.getMethod("getVersion").invoke(coreV2Instance);
String v1AltStr = (String) coreV1AltClass.getMethod("getVersion").invoke(coreV1AltInstance);
String v2AltStr = (String) coreV2AltClass.getMethod("getVersion").invoke(coreV2AltInstance);
String v3Str = coreV3Instance.getVersion();
String v3AltStr = coreV3AltInstance.getVersion();
// get version of dependency
String v1DepStr = (String) coreV1Class.getMethod("getDependencyVersion").invoke(coreV1Instance);
String v2DepStr = (String) coreV2Class.getMethod("getDependencyVersion").invoke(coreV2Instance);
String v1AltDepStr = (String) coreV1AltClass.getMethod("getDependencyVersion").invoke(coreV1AltInstance);
String v2AltDepStr = (String) coreV2AltClass.getMethod("getDependencyVersion").invoke(coreV2AltInstance);
String v3DepStr = coreV3Instance.getDependencyVersion();
String v3AltDepStr = coreV3AltInstance.getDependencyVersion();
System.out.println(String.format("V1 loader :: version = '%s' :: dependency_version = '%s'", v1Str, v1DepStr));
System.out.println(String.format("V2 loader :: version = '%s' :: dependency_version = '%s'", v2Str, v2DepStr));
System.out.println(String.format("V3 loader :: version = '%s' :: dependency_version = '%s'", v3Str, v3DepStr));
System.out.println(String.format("V1 custom loader :: version = '%s' :: dependency_version = '%s'", v1AltStr, v1AltDepStr));
System.out.println(String.format("V2 custom loader :: version = '%s' :: dependency_version = '%s'", v2AltStr, v2AltDepStr));
System.out.println(String.format("V3 custom loader :: version = '%s' :: dependency_version = '%s'", v3AltStr, v3AltDepStr));
NOTE: Core interface could be part of another project which is used by projects v1, v2, v3, which could allow us to cast the newly created instance and work in a typesafe manner. But this might not be doable every time.
