ClassLoader Summary

Here is ClassLoader summary inside ClassLoader.js, for more information, please also read the ClassLoader source.

ClassLoader creates SCRIPT elements and setup class path and onload callback to continue class loading.

In the onload callbacks, ClazzLoader will try to calculate the next-to-be-load *.js and load it. In *.js, it will contains some codes like
Clazz.load (…, “$wt.widgets.Control”, …);
to provide information to build up the class dependency tree.

Some known problems of different browsers:

1. In IE, loading *.js through SCRIPT will first triggers onreadstatechange event, and then executes inner *.js source.
2. In Firefox, loading *.js will first executes *.js source and then triggers onload event.
3. In Opera, similar to IE, but trigger onload event. (TODO: More details should be studied. Currently, Opera supports no multiple-thread-loading)

For class dependency tree, actually, it is not a tree. It is a reference net with nodes have n parents and n children. There is a root, which ClassLoader knows where to start searching and loading classes, for such a net. Each node is a class. Each class may require a set of must-classes, which must be loaded before itself getting initialized, and also need a set
of optional classes, which also be loaded before being called.

The class loading status will be in 6 stages.

1. Unknown, the class is newly introduced by other class.
2. Known, the class is already mentioned by other class.
3. Loaded, *.js source is in memory, but may not be initialized yet. It requires all its must-classes be intiailized, which is in the next stage.
4. Musts loaded, all must classes is already loaded and declared.
5. Declared, the class is already declared (ClazzLoader#isClassDefined).
6. Optionals loaded, all optional classes is loaded and declared.

The ClassLoader tries to load all necessary classes in order, and intialize them in order. For such job, it will traverse the dependency tree, and try to next class to-be-loaded. Sometime, the class dependencies may be in one or more cycles, which must be broken down so classes is loaded in correct order.

Loading order and intializing order is very important for the ClassLoader. The following technical options are considered:

1. SCRIPT is loading asynchronously, which means controling order must use callback methods to continue.
2. Multiple loading threads are later introduced, which requires the ClassLoader should use variables to record the class status.
3. Different browsers have different loading orders, which means extra tests should be tested to make sure loading order won’t be broken.
4. Java2Script simulator itself have some loading orders that must be honored, which means it should be integrated seamlessly to Clazz system.
5. Packed *.z.js is introduced to avoid lots of small *.js which requires lots of HTTP connections, which means that packed *.z.js should be treated specially (There will be mappings for such packed classes).
6. *.js or *.css loading may fail according to network status, which means another loading try should be performed, so ClazzLoader is more robust.
7. SWT lazy loading is later introduced, which means that class loading process may be paused and should be resumed later.