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?
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4 Answers
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" +
"};");
});
9 Comments
tyteen4a03
Doesn't seem to be working. My calls to
console.log() isn't being picked up for some reason.
SleepyFox89
Not sure if we need to do this every time state changes - perhaps just when
newValue == State.SCHEDULED or something? otherwise we are overriding console.log multiple times for no reason, in the same request.
Andrew Head
If this silently fails for you like it did for me, one thing to try is initializing
bridge in a persistent scope (e.g., as an member variable of an object, instead of a local variable). I think my bridge was garbage collected before it could be called from JavaScript (also see stackoverflow.com/a/41903154 for details about setMember and garbage collection)
Gili
@AndrewHead I fixed the answer. Thank you for pointing his out!
abulka
I was stumped since
import netscape.javascript.JSObject; is deprecated in Java 9 and import jdk.nashorn.api.scripting.JSObject; is deprecated in Java 15, with no replacements mentioned anywhere - I am using Java 20. Finally discovered that if you add requires jdk.jsobject; to your src/main/java/module-info.java then you can access import netscape.javascript.JSObject;. Not sure if this is kosher but it works. |
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 + "]");
});
6 Comments
vladsch
Yes, but you don't get the ability to expand and view the arguments passed to console methods or jump to source like you get from the debugger console.
dzikoysk
He just asked about getting output from console.log, so it's better alternative to Green Man's answer. Dedicated debugging tools belong to another case and it is not always helping - for example if we want to catch output in our final distributed app.
Guillaume F.
Sadly WebConsoleListener is not available in recent versions of Java.
FARS
@GuillaumeF. package com.sun.javafx.webkit; but it is not event from java, I'm using the lastest javaFX version, it is here
tresf
As the comments predicted.... sadly... this stopped working in newer FX versions :(
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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.
Comments
Thanks to Gili for his answer which taught me how to get this working in my code. My modifications to his solution I think are worth sharing though because it is a little less code and it redirects both console.log and console.error:
webEngine.getLoadWorker().stateProperty().addListener((ob, o, n) -> {
JSObject window = (JSObject) webEngine.executeScript("window");
window.setMember("javaOut", System.out);
window.setMember("javaErr", System.err);
webView.getEngine().executeScript(
"console.log = function(message) { javaOut.println(message); };\n"
+ "console.error = function(message) {javaErr.println(message); };");
});
If you want to do more than just redirect to standard out/error (like outputting to some GUI console or adding timestamps) then making something like Gili's JavaBridge class is probably worthwhile, but for a simple re-direct to standard out/err, this seems easier to me.
I did notice one limitation because I had some javascript code that did console.error('And error occured', error) which of course didn't work because I only overwrote the version of console.error() that only takes one argument. It was good enough for my purposes though.
Comments
