Java IO.pdf - Nguyen Dang Binh

9.5.7 Manifest
Java I/O
What the java.util.jar classes add to the superclasses in java.util.zip is the ability to
read the attributes of each JAR entry as well as the manifest for the entire JAR archive. Recall
that a JAR archive should have exactly one manifest file. That manifest file has entries that
apply to the entire file as well as entries for some (though perhaps not all) of the files stored in
the archive. Although physically the manifest file belongs to the entire archive, logically parts
of it belong to different entries in the archive. The java.util.jar.Manifest class represents
this manifest file.
public class Manifest extends Object implements Cloneable // Java 2
It has methods to get the entries and attributes of a manifest, to write the manifest onto an
output stream or to read entries from an input stream, and an assortment of utility methods.
The Manifest class has three constructors:
public Manifest() // Java 2
public Manifest(InputStream in) throws IOException // Java 2
public Manifest(Manifest man) // Java 2
The first constructor creates an empty manifest (one with no entries); the second reads the
manifest from the given stream; the third copies the manifest from the Manifest object
passed as an argument. However, all three are mostly for the internal use of Java. Instead,
client programmers use the getManifest() method of JarFile to retrieve the Manifest
object for the manifest file in a particular archive. For example:
JarFile jf = new JarFile("classes.jar");
Manifest m = jf.getManifest();
The Manifest class has three methods that return a map of the entries in the manifest.
getEntries() returns an unspecified Map (a HashMap object in JDK 1.2) in which the keys
are the entry names and the values are the Attributes objects for the entry:
public Map getEntries() // Java 2
The getMainAttributes() method returns an Attributes object representing the attributes
in the manifest file that apply to the file as a whole rather than to any individual entry in the
file, such as Manifest-Version:
public Attributes getMainAttributes() // Java 2
The getAttributes() method returns an Attributes object containing the name/value pairs
of the named entry. The Name attribute is not included in this list:
public Attributes getAttributes(String entryName) // Java 2
The clear() method removes all entries and attributes from the manifest so that it can be
reused; client programmers have little reason to call this method:
public void clear() // Java 2
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