Java Reflection as Android API

Have you ever tried or wished to re-use or access source code which is part of a different Android APK ? If "Yes", then this article is for you.

Well, I must agree its not a very common use case and when it comes to API in Android, we rather prefer JAR or Remote Service to expose functionalists. Java Reflection can be an alternate and useful API approach to expose functionalists from an APK or Application.

Here I'm going to explain a design approach where we can define API for Android using Java Reflection. For this example, Host.apk is an Application which hosts one API method which we are going to access from another application Client.apk.

Host.apk includes- HostApi.java which defines API method getMeta(). This method returns some pre-defined Text appended to calling Application's Package name.

package com.pras.host;

import android.content.Context;

public class HostApi {

     Context mContext;

     public HostApi(Context context) {
         this.mContext = context;
     }

     public String getMeta() {
         String pkgName = mContext.getPackageName();
         return "HostKey-" + pkgName;
     }
}

To access above API method getMeta(), we need to write a HostApi Proxy which access HostApi.java using Reflection. Here is the code snippet-

package com.pras.client; 

import java.lang.reflect.Constructor; 
import java.lang.reflect.Method; 
import android.content.Context;

public class HostApiProxy {

     private final String REMOTE_PACKAGE = "com.pras.host";
     private final String REMOTE_CLASS = "com.pras.host.HostApi";
     private final String REMOTE_METHOD = "getMeta";

     Context mContext;

     public HostApiProxy(Context context) {
         this.mContext = context;
     }

     public String getMeta() {
         String metaData = null;
         try {
              Class hostApiClass = getRemoteClass(mContext, REMOTE_PACKAGE,
                       REMOTE_CLASS);
              Object hostApiInstance = getRemoteInstance(hostApiClass,
                       new Class[] { Context.class }, new Object[] { mContext });
              metaData = (String) invokeRemoteMethod(hostApiInstance,
                       REMOTE_METHOD, new Class[] {}, new Object[] {});
         } catch (Exception ex) {
              ex.printStackTrace();
         }
         return metaData;
     }

     public Class getRemoteClass(Context context, String remotePackage,
              String remoteClass) throws Exception {

         // Get remote Application Context and request to include Source
         Context remoteContext = context.createPackageContext(remotePackage,
                  Context.CONTEXT_IGNORE_SECURITY | Context.CONTEXT_INCLUDE_CODE);
         // Get Class loader associated to Remote Context
         ClassLoader loader = remoteContext.getClassLoader();
         Class cls = loader.loadClass(remoteClass);
         return cls;
     }

     public Object getRemoteInstance(Class remoteClass, Class[] args,
              Object[] argValues) throws Exception {
         Constructor constructor = remoteClass.getConstructor(args);
         return constructor.newInstance(argValues);
     }

     public Object invokeRemoteMethod(Object instance, String methodName,
              Class[] args, Object[] argValues) throws Exception {
         Method method = instance.getClass().getDeclaredMethod(methodName, args);
         // Make sure its accessible (if it is Private method)
         method.setAccessible(true);
         return method.invoke(instance, argValues);
     }
}

I have defined 3 utility methods- getRemoteClass(), getRemoteInstance() and invokeRemoteMethod() which help to get reference of remote Class stored in Host.apk and invoke getMeta() method. The important point to note here; to load the remote class, we need to get the instance of Class Loader of the respective Application's context. In Android, each application Context has its own Class Loader which is responsible to load/manage classes included inside the Application or APK.

Above approach and Reflection methods can be used to define any API. So, if you like this design approach, give a 2nd thought during designing your next API. Please find the complete Source and APK (Host.apk and Client.apk).