Calling Python from Java for Android using CLE and SL4A

Java is major language to develop applications on Android platform. But, in some cases, programmers want to call Python code from Java to perform some functions.

By now, there is no direct method to write programs with Python.

SL4A is an open project, which supports script languages including python, lua, etc. But calling python from Java has not been supported directly. Programmers have to use JNI method, and native code to write interface for python. This is more difficult, you have to manage relationship between Java objects and python objects, kinds of references, interface parameters conversion, and callback functions from python to Java.

CLE(Common Language Extension) is a middleware which presents functions to aid multi-language calls between scripts, including Java and python. Using CLE, the above problems can be solved easily. Architecture of CLE is as follows:

Java calls Python functions:

Python Function

Obj=Service._New("TestClass");
def Obj_PythonAdd(self,x,y) :
    return x+y;
Obj.PythonAdd = Obj_PythonAdd;

Java Code

StarObjectClass a = Service._GetObject("TestClass")._New();
a._Call("PythonAdd",12,34));
Callback of python to java:

Java Callback Function

StarObjectClass a = Service._GetObject("TestClass")._New()._Assign(new StarObjectClass(){
   public int JavaAdd(StarObjectClass self,int x,int y){
       return x+y;
   }
});

Python Code

Obj.JavaAdd(x,y)

Java gets object’s attributes defined in python.

Python: Obj.PythonValue = 200;

Java: a._Get("PythonValue")
Python gets object’s attributes defined in Java.

Java: a._Set("JavaValue",100);
Python: self.JavaValue

Method1 Source Code (Integrate CLE With Your Project)

1. Install SL4A from http://code.google.com/p/android-scripting/downloads/list
2. Download devfiles from http://code.google.com/p/cle-for-android, and then Open Eclipse
3. Create project for Android.
4. Add CLE libraries to project as follows:

   

   

5. Python code:

   SrvGroup = libstarpy._GetSrvGroup()
   
   Service = SrvGroup._GetService("","")
   
   #Create objects
   Obj=Service._New("TestClass");
   
   #Define functions
   def Obj_PythonAdd(self,x,y) :
       print("Call python function...");
       return x+y;
   Obj.PythonAdd = Obj_PythonAdd;
   
   #Call java functions
   def Obj_PythonPrint(self,x,y) :
       print( "Value defined in java is ",self.JavaValue );
       print( "Function result from java ",self.JavaAdd(x,y) );
   Obj.PythonPrint = Obj_PythonPrint;
   
   #define Attributes
   Obj.PythonValue = 200;

6. Java code:
   Java

   package com.cle.pythonfromjava;
   
   import android.app.Activity;
   import android.os.Bundle;
   import android.content.res.AssetManager;
   
   import java.io.IOException;
   import java.io.InputStream;
   
   import com.srplab.www.starcore.*;
   
   public class PytonfromjavaActivity extends Activity {
       /** Called when the activity is first created. */
       @Override
       public void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
           setContentView(R.layout.main);
           
           //--init CLE
           StarCoreFactoryPath.InitDefault(Runtime.getRuntime(),
   			"/data/data/com.cle.pythonfromjava");
           
   StarCoreFactory starcore= StarCoreFactory.GetFactory();
   StarServiceClass Service=starcore._InitSimple("test","123",0,0);
   Service._CheckPassword(false);
   
   AssetManager assetManager = getAssets();     
   
           try{
               String pythonbuf;
               
                  InputStream dataSource = assetManager.open("code.py");
                  int size=dataSource.available();
                  byte[] buffer=new byte[size]; 
                  dataSource.read(buffer); 
                  dataSource.close();        
                  pythonbuf=new String(buffer);
                  
               Service._RunScript("python",pythonbuf,"cmd","");
           }
           catch(IOException e ){
           }   
           
   StarObjectClass a = Service._GetObject("TestClass")._New()._Assign(new StarObjectClass(){
      public int JavaAdd(StarObjectClass self,int x,int y){
          System.out.println("Call java function...");
          return x+y;
      }
   });
           a._Set("JavaValue",100);
           System.out.println(a._Get("PythonValue")); 
           System.out.println(a._Call("PythonAdd",12,34));           
           a._Call("PythonPrint",56,78);
       }
   
   }

Method2 Source Code (Install cle from http://code.google.com/p/cle-for-android independently):

1. Install SL4A from http://code.google.com/p/android-scripting/downloads/list
2. Install CLE from http://code.google.com/p/cle-for-android/downloads/list
3. Download devfiles from http://code.google.com/p/cle-for-android, and then Open Eclipse
4. Create project for Android
5. Python code:
   Same as method 1
6. Java code:
   Delete the following code line. Others are the same as method1.
   Java

   StarCoreFactoryPath.InitDefault(Runtime.getRuntime(),
   			"/data/data/com.cle.pythonfromjava");

Examples may be downloaded from http://www.srplab.com/android/calling_python_from_java.rar.