20191008173421

20191008173421

***Java2Script: How It Works***

**java2script/SwingJS**

The full java2script/SwingJS operation involves two parts: Creating the JavaScript from the abstract syntax tree (java2script), and running that within the browser (SwingJS). Both are discussed below.

The code for these two parts are well-separated:

net.sf.j2s.core java2script transpiler

net.sf.j2s.java.core SwingJS runtime

[Note: You may notice that the java2script project includes several other net.sf.... projects. Frankly, I have no idea what they are for. My guess is that they don't work. I think perhaps they were early attempts to get all this working within Eclipse at runtime, with "hot" connections to code. But that never ever worked for me, and what we have now -- direct creation of a site directory that can be debugged in a standard external browser is way better, anyway. I have left them there just because I haven't taken the time to get rid of them.]

**java2script transpiler**

[Note: Changes in java2script should be necessary only when core Java syntax changes or is supplemented in the core Java. For example, Java 8 allows switch cases that are String constants, while Java 6 does not. When we went to Java 8, we had to modify Java2ScriptVisitor.java to account for that. If ever there is a need to fix something that the java2script compiler is doing wrong or to adapt to new Java syntax, as for Java 11, look in net.sf.j2s.core.Java2ScriptVisitor.]

[Note that a big difference between Java and JavaScript is that JavaScript functions cannot be "overloaded", meaning the two calls in JavaScript:

write(b)

write(b, 2, 3)

will both point to the same function. A major task of the java2script transpiler is to sort this out before it becomes an issue at runtime. It does this by creating signature-specific function names in JavaScript, such as write$B and write$B$I$I.]

**Creating a New Transpiler**

The transpiler is created in Eclipse by checking out the net.sf.j2s.core project from GitHub as a standard Java project and adjusting the code as necessary. When it is desired to create the transpiler (net.sf.j2s.core.jar):

1) Use File...Export...Deployable plug-ins and fragments

(if you do not see this option, check that you are using Eclipse

Enterprise)

2) Choose net.sf.j2s.core (Version 3.2.4), check the directory,

and press finish.

3) Copy this file to the drop-ins directory, restart Eclipse,

and test.

4) Copy this file to the project dist/swingjs folder and also

to the swingjs/ver/3.2.4 folder (or appropriate).

I do this with a DOS batch file, which also adds a timestamp.

That's it. I advise to NOT change the version number. I know, this

sounds stupid, but if you change that number, installation in Eclipse requires starting Eclipse with a -clean as the first option. https://www.eclipsezone.com//eclipse/forums/t61566.html

This is pain. If you do not do this clean build, Eclipse just

ignores your drop-in.

The dist directory also includes SwingJS-site.zip, created from the net.sf.j2s.core.java project.

**SwingJS runtime maintenance**

[Note: As of 10/2019, SwingJS is still a collection of Java code from a variety of sources. There is some original Apache Java code from before 2016, and there is code that is a mix of Java6 and Java8. Some of the core java.lang classes are created in j2sClazz.js directly, disregarding the class files in src. I realize this makes maintenance challenging, but that is the way it is. It's a volunteer operation....]

As Java evolves, new packages, classes, and methods are added. Some classes are deprecated and perhaps even removed. For example, the java.util.stream package was added in Java 8, and along with that, several core java.lang classes and interfaces (such as java.lang.String) were augmented to include stream-based methods. There have also been a number of cases where generic classes have been converted to typed classes. For example, staring in Java 7, javax.swing.JComboBox became javax.swing.JComboBox<E>. To date, this has not been an issue. (Particularly in the area of generics, java2script uses an aliasing scheme to refer to the same method by different names so that code from different Java versions still runs appropriately. -- For example, java.util.Calendar.compareTo is aliased as

compareTo$java\_util\_Calendar, compareTo$, and compareTo$TT

Java 11 makes a huge leap in the handling of String and char. SwingJS will have to be maintained to keep up with these changes.

The important thing here is to maintain backward compatibility at all times. It is critical that any changes to SwingJS do not break the running of code from older versions of Java. We do not have the luxury in SwingJS of packaging code with its own JRE (other than the obvious fact that you can put whatever you want into site/swingjs/j2s on your own web site).

So, basically, you can add to or modify any of the several thousand classes in j2s.net.sf.java.core/src. Just be careful to allow for what is already there to still run. Adding a new class can be a challenge. Note that we have not implemented serialization or accessibility. This was a design decision based on need. We needed applet code to run, and these two features added far too much complexity to the task. So we ignored them. (Serialization, in particular, is probably not ever going to work in JavaScript, because the java2script transpiler does not preserve enough information about variable types to make that work properly.)

Bob Hanson 2019.10.13

/**

public void setLocationRelativeTo(Component c) {

Container root = null;

if (c != null) {

if (c.isWindowOrJSApplet()) {

root = (Container) c;

} else {

Container parent;

for (parent = c.getParent(); parent != null; parent = parent.getParent()) {

if (parent.isWindowOrJSApplet()) {

root = parent;

break;

}

}

}

}

if ((c != null && !c.isShowing()) || root == null || !root.isShowing()) {

Dimension paneSize = getSize();

Point centerPoint = GraphicsEnvironment.getLocalGraphicsEnvironment().getCenterPoint();

setLocation(centerPoint.x - paneSize.width / 2, centerPoint.y - paneSize.height / 2);

} else {

Dimension invokerSize = c.getSize();

Point invokerScreenLocation = c.getLocationOnScreen();

Rectangle windowBounds = getBounds();

int dx = invokerScreenLocation.x + ((invokerSize.width - windowBounds.width) >> 1);

int dy = invokerScreenLocation.y + ((invokerSize.height - windowBounds.height) >> 1);

Rectangle ss = root.getGraphicsConfiguration().getBounds();

// Adjust for bottom edge being offscreen

if (dy + windowBounds.height > ss.y + ss.height) {

dy = ss.y + ss.height - windowBounds.height;

if (invokerScreenLocation.x - ss.x + invokerSize.width / 2 < ss.width / 2) {

dx = invokerScreenLocation.x + invokerSize.width;

} else {

dx = invokerScreenLocation.x - windowBounds.width;

}

}

// Avoid being placed off the edge of the screen

if (dx + windowBounds.width > ss.x + ss.width) {

dx = ss.x + ss.width - windowBounds.width;

}

if (dx < ss.x)

dx = ss.x;

if (dy < ss.y)

dy = ss.y;

setLocation(dx, dy);

}

}

@Override

@Override

@Override

@Override

@Override

public void flush() {

// call this method after drawing to ensure that

// pixels are recreated from the HTML5 canvas

秘pix = null;

秘havePix = false;

// was for surfaceManager only super.flush();

}

// 秘pix = null;

flush(); // also setting 秘havePix false

import java.awt.Point;

/**

@Override

public Point getLocationOnScreen() {

return (isShowing() ? getRootPane().getLocationOnScreen() : null);

}