SAX概念
SAX是Simple API for XML的缩写，它并不是由W3C官方所提出的标准，可以说是“民间”的事实标准。实际上，它是一种社区性质的讨论产物。虽然如此，在XML中对SAX的应用丝毫不比DOM少，几乎所有的XML解析器都会支持它。
与DOM比较而言，SAX是一种轻量型的方法。我们知道，在处理DOM的时候，我们需要读入整个的XML文档，然后在内存中创建DOM树，生成 DOM树上的每个Node对象。当文档比较小的时候，这不会造成什么问题，但是一旦文档大起来，处理DOM就会变得相当费时费力。特别是其对于内存的需 求，也将是成倍的增长，以至于在某些应用中使用DOM是一件很不划算的事（比如在applet中）。这时候，一个较好的替代解决方法就是SAX。
SAX在概念上与DOM完全不同。首先，不同于DOM的文档驱动，它是事件驱动的，也就是说，它并不需要读入整个文档，而文档的读入过程也就是 SAX的解析过程。所谓事件驱动，是指一种基于回调（callback）机制的程序运行方法。（如果你对Java新的代理事件模型比较清楚的话，就会很容 易理解这种机制了）
在XMLReader接受XML文档，在读入XML文档的过程中就进行解析，也就是说读入文档的过程和解析的过程是同时进行的，这和DOM区别很 大。解析开始之前，需要向XMLReader注册一个ContentHandler，也就是相当于一个事件监听器，在ContentHandler中定义 了很多方法，比如startDocument()，它定制了当在解析过程中，遇到文档开始时应该处理的事情。当XMLReader读到合适的内容，就会抛 出相应的事件，并把这个事件的处理权代理给ContentHandler，调用其相应的方法进行响应。
The ASM library provides two APIs for generating and transforming compiled classes: the core API provides an event based representation of classes, while the tree API provides an object based representation.
With the event based model a class is represented with a sequence of events, each event representing an element of the class, such as its header, a field, a method declaration, an instruction, etc. The event based API defines the set of possible events and the order in which they must occur, and provides a class parser that generates one event per element that is parsed, as well as a class writer that generates compiled classes from sequences of such events.
With the object based model a class is represented with a tree of objects, each object representing a part of the class, such as the class itself, a field, a method, an instruction, etc. and each object having references to the objects that represent its constituents. The object based API provides a way to convert a sequence of events representing a class to the object tree representing the same class and, vice versa, to convert an object tree to the equivalent event sequence. In other words the object based API is built on top of the event based API.
These two APIs can be compared to the Simple API for XML (SAX) and Document Object Model (DOM) APIs for XML documents: the event based API is similar to SAX, while the object based API is similar to DOM. The object based API is built on top of the event based one, like DOM can be provided on top of SAX.
ASM provides both APIs because there is no best API. Indeed each API has its own advantages and drawbacks:
-
The event based API is faster and requires less memory than the object based API, since there is no need to create and store in memory a tree of objects representing the class (the same difference also exists between SAX and DOM).
-
However implementing class transformations can be more difficult with the event based API, since only one element of the class is available at any given time (the element that corresponds to the current event), while the whole class is available in memory with the object based API.
Note that the two APIs manage only one class at a time, and independently of the others: no information about the class hierarchy is maintained, and if a class transformation affects other classes, it is up to the user to modify these other classes.