Java IO.pdf - Nguyen Dang Binh

Java I/O
Again, these do not change the data in any way and may be used exactly like any other
write() method. They simply update the internal checksum with the bytes written as they're
written. Both CheckedOutputStream and CheckedInputStream have a getChecksum()
method that returns the Checksum object for the stream. You can use this Checksum object to
get the current value of the checksum for the stream.
public Checksum getChecksum()
These methods return a reference to the actual Checksum object that's being used to calculate
the checksum. It is not copied first. Thus, if a separate thread is accessing this stream, the
value in the checksum may change while you're working with the Checksum object.
Conversely, if you invoke one of this Checksum object's update() methods, it affects the
value of the checksum for the stream as well.
9.5 JAR Files
Java 1.1 added support for Java ARchive files, JAR files for short. JAR files bundle the many
different classes, images, and sound files an applet requires into a single file. It is generally
faster for a web browser to download one JAR file than to download the individual files the
archive contains, since only one HTTP connection is required. An applet stored in a JAR file,
instead of as merely loose .class files, is embedded in a web page with an tag with
an archive attribute pointing to the JAR file. For example:
The code attribute still says that the main class of this applet is called NavigationMenu.
However, a Java 1.1 web browser, rather than asking the web server for the file
NavigationMenu.class as a Java 1.0 web browser would, asks the web server for the file
NavigationMenu.jar. Then the browser looks inside NavigationMenu.jar to find the file
NavigationMenu.class. Only if it doesn't find NavigationMenu.class inside
NavigationMenu.jar does it then go back to the web server and ask for NavigationMenu.class.
Now suppose the NavigationMenu applet tries to load an image called menu.gif. The applet
will look for this file inside the JAR archive too. It only has to make a new connection to the
web server if it can't find menu.gif in the archive.
Sun wisely decided not to attempt to define a new file format for JAR files. Instead, they
stuck with the tried-and-true zip format. This means that the classes, images, sounds, and
other files stored inside a JAR archive can be compressed, making the applet even faster to
download. This also means that standard tools like PKZip and standard zip libraries like
java.util.zip can work with JAR files.
JAR files have also become Java's preferred means of distributing Java Beans and class
libraries. For instance, the Java Cryptography Extension, discussed in the next chapter, is
mostly a set of classes packed up in the file jce12-ea-dom.jar. Since the library is distributed
as a single file rather than a collection of nested folders, it's harder for one file to get moved or
deleted. The overall system is more robust. To make the files contained in the archive
available to Java, the complete path to the archive itself is added to the class path. For
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