How Java gets the classes?

In this article we are going to see how classes are found/work in JAVA with java tools. So, we will discussing about java, javac, javadoc tools which work directly with java classes and java source code.

As we know, first we make source code in to byte code(class file or jar file) and then run byte code.
Java Source ->Java Byte Code ->Running byte code in JVM.

Besides the source code generated class files, there are three type of classes.
1. User Classes , which are defined by developers(our made byte code). We can set location by -classpath option
2. Extension Classes, which are used by user classes or environments. These are 3rd party extensions that helps for a particular purpose. (usually as .jar files under jre/lib/ext folder)
3. Bootstrap classes, which comes with Java Run-time Environment(in rt.jar).

Security Factors : Class loaders loads Classes and interfaces to use them with a security policy. A program can load a class/interface by calling loadClass method of a class loader object. It calls internal class loader that applies the security policy. (I will provide separate post for security policy implementation example)
-If enabled, policy is configured by system and user policy files
-If policy is not enabled, by default all are "Trusted"
-Bootstrap classes are always "Trusted"

For Java command : (to run byte code)
When we run java launcher(java command), it first look for bootstrap classes, then extensions and then user classes. Java command reads system property (from sun.boot.class.path) and gets the bootstrap class first. We can redefine using -Xbootclasspath.(helpful when we need to run with different version of JRE)
Java command loads extensions using extension mechanism . Byte code must be in JAR files(loose class files will not be found). We can not change the location. Same name classes under different JARs are undefined.
For User class, Java command follow -classpath for JARs/Class files. The user class contains fully qualified names(with package name) and stored in tree formation folder followed by the name.(A dot will create a child folder. i.e. If a qualified class name = com.package.MyClass , then the my class will be com/package/ file. and byte code will be com/package/MyClass.class). Java command puts the user class path string in the java.class.path system property. where
- The default  ".", means that user class files are all the class files in the current directory (or under it, if in a package).
-The value of the CLASSPATH environment variable, which overrides the default value.
-The value of the -cp or -classpath option overrides both the default value and the CLASSPATH value.
-The JAR is specified by -jar option, which overrides all other values. If this option is used, all user classes must come from the specified archive.
 If , classes are in a JAR file, java command gets class list from manifest (I will provide a separate post for JAR file specification)
In general, classes referenced by a
-The JAR files in the JAR-class-path are searched after any earlier class path entries, and before any entries that appear later in the class path.
-If the JAR-class-path points to already searched JARs ,then these will not be searched again. (For  optimization and avoiding circular searches)
-If a JAR file is installed as an extension, then any JAR-class-path it defines is ignored.(it will not be consider as user class, will be handled like as other extensions are handled)

For Javac and Javadoc : (to compile source code)
-Like as java, javac and javadoc loads class files to run
-For source code processing both need object type used in source code.
-The class files are used to resolve source code references but,
-Both resolve references to classes and interfaces that may be present in class files, source files, or both (though have nothing to do with implementation of javac or javadoc)
-The tools classes in tools.jar are only used to run javac and javadoc.
-Both javac and javadoc supports bootstrap and extension class path changing with -bootclasspath and -extdirs options. (not modify to the classes that runs the javac or javadoc themselves).
-If a referenced class is found in both a class file and source file, javadoc always uses the source file but javac uses class files(automatically recompiles any out of date class files , these rules have specifications)
-By default, javac/javadoc search the user class path for both class and source unless -sourcepath is specified (for source files only)


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