A. Sundararajan's Weblog

4 ways to implement Java interface in JavaScript

With Mustang's JavaScript engine, there are 4 ways to implement Java interface in script:

1. JavaAdapter function

   
       var r = new JavaAdapter(java.lang.Runnable,
                  { 
                      run: function() { print('hello'); } 
                  });
       r.run();
   
   

   This feature may also be used with Java anonymous class-like syntax as shown below:

   
       var r = new java.lang.Runnable() { 
                  run: function() { 
                      print('hello') 
                  }
               };
       r.run();
   
   

   Unlike, Rhino's JavaAdapter, Mustang's JavaAdapter does not support implementing multiple Java interfaces. Also, extending a super class is not allowed.
2. using engine variable in jrunscript (or exposing ScriptEngine object as global variable and using it in script)

   
       var x = { run: function() { print('hello') } }
       var r = engine.getInterface(java.lang.Runnable, x);
       r.run();
   
   

3. pass script function whenever a Java interface type parameter is required! (this works from Rhino 1.6R2 onwards -- so, you have to wait for Mustang build 59 or above for this)

   
       function run() { 
           print('hello'); 
       }
       // script function is wrapped automatically
       var t = new java.lang.Thread(run); 
       t.start();   
   
   

   This feature works only if all the methods of the interface have the same signature or there is only one method in the interface. For interface with multiple methods, script function receives the name of the method as first parameter.
4. using JSAdapter and JavaAdapter.

   
       function Invoker(obj) {
           return new JSAdapter() {
               __has__: function(name) {
                   return true;
               },
               __get__: function(name) {
                   return function() {
                       return obj.invoke(name, arguments);
                   }
               }
           }
       }
   
       var r = { 
              invoke: function(name, args) { 
                  print(name + " called"); 
              }
           };
       var x = new JavaAdapter(java.lang.Runnable, Invoker(r));
       x.run(); // this calls r.invoke('run');
   
   

   This scheme even if interface methods don't have identical signature. But, invoke method has to handle variable number of arguments passed as args array (second argument). The first argument is the name of the interface method.

( Nov 01 2005, 08:28:51 AM IST ) Permalink | | | | |