JavaFX 8 WebEngine: How to get console.log() from javascript to System.out in java?

I'm using both JavaFX and the javascript engine inside JavaFX WebEngine to develop an application. I'd like to get feedback from javascript for debugging purposes. What happens to the console output inside the WebEngine? Is there any way I can access it, or redirect to System.out in java?

Solution 1:

The following code redirects console.log() to JavaBridge.log():

import netscape.javascript.JSObject;

[...]

public class JavaBridge
{
    public void log(String text)
    {
        System.out.println(text);
    }
}

// Maintain a strong reference to prevent garbage collection:
// https://bugs.openjdk.java.net/browse/JDK-8154127
private final JavaBridge bridge = new JavaBridge();

[...]

webEngine.getLoadWorker().stateProperty().addListener((observable, oldValue, newValue) ->
{
    JSObject window = (JSObject) webEngine.executeScript("window");
    window.setMember("java", bridge);
    webEngine.executeScript("console.log = function(message)\n" +
        "{\n" +
        "    java.log(message);\n" +
        "};");
});

Solution 2:

You can just add message listener to see what's happening in your output. You don't have to inject js bridge with redefinition of functions like console.log for every single loaded page

WebConsoleListener.setDefaultListener((webView, message, lineNumber, sourceId) -> {
    System.out.println(message + "[at " + lineNumber + "]");
});

Solution 3:

I like to go the other direction. We use log4j so I created a javascript wrapper like the following:

module.exports = {

    levels:[ "ALL", "TRACE", "DEBUG", "INFO", "WARN", "ERROR", "OFF"],

    level:"INFO",

    error:function(msg){
      if(this.isErrorEnabled()){
        console.error(msg)
      }
    },
    warn:function(msg){
      if(this.isWarnEnabled()){
        console.warn(msg)
      }
    },
    info:function(msg){
      if(this.isInfoEnabled()){
        console.log("INFO: "+msg)
      }
    },
    debug:function(msg){
      if(this.isDebugEnabled()){
        console.log("DEBUG: "+msg)
      }
    },
    trace:function(msg){
      if(this.isTraceEnabled()){
        console.log("TRACE: "+msg)
      }
    },

    isErrorEnabled:function(){
      return this.isEnabled("ERROR");
    },
    isWarnEnabled:function(){
      return this.isEnabled("WARN");
    },
    isInfoEnabled:function(){
      return this.isEnabled("INFO");
    },
    isDebugEnabled:function(){
      return this.isEnabled("DEBUG");
    },
    isTraceEnabled:function(){
      return this.isEnabled("TRACE");
    },
    isEnabled:function(statementLevel){
      return this.levels.indexOf(this.level)<=this.levels.indexOf(statementLevel);
    }
  }

Then at the beginning of the javascript I check to see if the log is present and set it:

if(window.log == undefined){
  window.log = require("./utils/log4j-wrapper")
  window.log.level = "INFO"
}

And that way if you set the Log4j logger directly on the engine before you even load the url like:

WebEngine webEngine = webView.getEngine()
JSObject win = (JSObject) webEngine.executeScript("window")
win.setMember("log", log)  //log being the java log4j logger

This way I can get logging in if I am opening directly in a browser or it is being run from a WebView in a JavaFX program. And has the added benefit of having levels to the logging in javascript that match your packages of the WebView controller. Just an alternative for larger javascript views.