how to use java to get a js file and execute it and then get the result

How can开发者_JAVA技巧 I use java to get a js file located on a web server, then execute the function in the js file and get the result and use the result in java.

Can you guys give me some code snippet? Great thanks.

You can use the scripting engine built into Java:

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;

public static void main(String[] args) throws Exception {
    ScriptEngineManager mgr = new ScriptEngineManager();
    ScriptEngine engine = mgr.getEngineByName("JavaScript");

    Object result = engine.eval("my-java-script-code") 
    System.out.println("Result returned by Javascript is: " + result);
}

Here is a more elaborate example.

There's three steps to this process:

1. Fetch the JS file from the server.
2. Execute some JS function from the file.
3. Extract the result.

The first step is fairly simple, there are lots of HTTP libraries in Java that will do this - you effectively want to emulate the simple functionality of something like wget or curl. The exact manner in which you do this will vary depending on what format you want the JS file in for the next step, but the process to get hold of the byte stream is straightforward.

The second step will require executing the JS in a Javascript engine. Java itself cannot interpret Javascript, so you'd need to obtain an engine to run it in - Rhino is a common choice for this. Since you'd need to run this outside of Java, you'll likely have to spawn a process for execution in Rhino using ProcessBuilder. Additionally, depending on the format of the Javascript you might need to create your own "wrapper" javascript that functions like a main class in Java and calls the method in question.

Finally you need to get the result out - obviously you don't have direct access to JavaScript objects from your Java program. The easiest way is going to be for the JS program to print the result to standard out (possibly serialising as something like JSON depending on the complexity of the object), which is being streamed directly to your Java app due to the way you launched the Rhino process. This could be another job for your JS wrapper script, if any. Otherwise, if the JS function has observable side effects (creates a file/modifies a database) then you'll be able to query those directly from Java.

Job done.

I hope you realise this question is far too vague to get full answers. Asking the public to design an entire system goes beyond the point where you'll get useful, actionable responses.

There are plenty of examples on the web of how to download a file from a URL.

Suns version of the JDK and JRE includes the Mozilla Rhino scripting engine.

Assuming you have stored the contents of the javascript file in a string called 'script', you can execute scripts as follows

 String result = null;
 ScriptEngineManager mgr = new ScriptEngineManager();
  ScriptEngine jsEngine = mgr.getEngineByName("JavaScript");
  try {
    jsEngine.eval(script);
    result = jsEngine.get("result")
  } catch (ScriptException ex) {
      ex.printStackTrace();
  } 

The result will be extracted from the engine and stored in the 'result' variable.

The is a tutorial on scripting in Java that might be useful.