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Java in a Nutshell

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JDK Tools



java---The Java Interpreter


JDK 1.0 and later.


java [ interpreter options ] classname [ program arguments ]
java_g [ interpreter options ] classname [ program arguments ]


java is the Java byte-code interpreter--it runs Java programs. java_g is a debugging version of the interpreter. It is unoptimized, and has some additional options for tracing the execution of a program.

The program to be run is the class specified by classname. This must be a fully qualified name, it must include the package name of the class, but not the .class file extension. Note that you specify the package and class name, with components separated by '.', not the directory and filename of the class, which has its components separated by '/' or '/'. If a Java class has no package statement, then it is not in any package, and the class name is specified alone. Examples:

% java david.games.Checkers
% java test

See the description of the -classpath option and the CLASSPATH environment variable below for information on specifying where java should look for classes.

The class specified by classname must contain a method main() with exactly the following signature:

public static void main(String argv[])

Any arguments following the classname on the java command line are placed into an array and passed to the main() method when java starts up.

If main() creates any threads, java runs until the last thread exits. Otherwise, the interpreter executes the body of main() and exits.

Although only a single class name is specified when invoking java, the interpreter automatically loads any additional classes required by the program. These classes are located relative to the Java class path, described under the -classpath option below.

By default, java runs a byte-code verifier on all classes loaded over the network. This verifier performs a number of tests on the byte-code of the loaded class to ensure, for example, that it does not corrupt the internal operand stack and that it performs appropriate run-time checks on such things as array references. The -verify, -noverify, and -verifyremote options control the byte-code verification process.


-classpath path

The path that java uses to look up the specified classname and all other classes that it loads. Specifying this option overrides the default path and the CLASSPATH environment variable. The class path is an ordered list of directories and ZIP files within and below which java searches for named classes. On UNIX systems, a path is specified as a colon-separated list of directories and ZIP files. On Windows systems, directories and ZIP files (which may have drive specifiers that use colons) are separated from each other with semicolons. For example, a UNIX -classpath specification might look like this:

-classpath /usr/lib/java/classes:.:~/java/classes

On a Windows system, the specification might be:

-classpath C:\tools\java\classes.zip;.;D:\users\david\classes

A period by itself in the path indicates that the current working directory is searched. Directories and ZIP files are searched in the order they appear. Place the standard Java classes first in the path if you do not want them to be accidentally or maliciously overridden by classes with the same name in other directories.

java expects to find class files in a directory hierarchy (or with a directory name within a ZIP file) that maps to the fully qualified name of the class. Thus, on a UNIX system, Java would load the class java.lang.String by looking for the file java/lang/String.class beneath one of the directories specified in the class path. Similarly, on a Windows 95 or Windows NT system (which support long filenames), java would look for the file java\lang\String.class beneath a specified directory or within a specified ZIP file.

If you do not specify -classpath or the CLASSPATH environment variable, the default class path is:

.:$JAVA/classes:$JAVA/lib/classes.zip       UNIX systems
.;$JAVA\classes;$JAVA\lib\classes.zip       Windows systems

Where $JAVA is JDK installation directory.

-cs, -checksource

Both of these options tell java to check the modification times on the specified class file and its corresponding source file. If the class file cannot be found or if it is out of date, it is automatically recompiled from the source.


Defines propertyname to equal value in the system properties list. Your Java program can then look up the specified value by its property name. You may specify any number of -D options. For example:

% java -Dawt.button.color=gray -Dmy.class.pointsize=14 my.class


Causes java to display a password as it starts up. This password can be used to allow the jdb debugger to attach itself to this interpreter session. Note that this password should not be considered cryptographically secure.


Print a usage message and exit.


Sets the logging level for trace output. java_g only.

-ms initmem[k|m]

Specifies how much memory is allocated for the heap when the interpreter starts up. By default, initmem is specified in bytes. You can specify it in kilobytes by appending the letter k or in megabytes by appending the letter m. The default is 1 MB. For large or memory intensive applications (such as the Java compiler), you can improve run-time performance by starting the interpreter with a larger amount of memory. You must specify an initial heap size of at least 1000 bytes.

-mx maxmem[k|m]

Specifies the maximum heap size the interpreter will use for dynamically allocated objects and arrays. maxmem is specified in bytes by default. You can specify maxmem in kilobytes by appending the letter k and in megabytes by appending the letter m. The default is 16 MB. You must not specify a heap size less than 1000 bytes.


Do not do garbage collection asynchronously. With this option specified, java only performs garbage collection when it runs out of memory or when the garbage collector is explicitly invoked. Without this option, java runs the garbage collector as a separate, low-priority thread.


Do not garbage collect loaded classes that are no longer in use. This option is only available in JDK 1.1 and later.


Never run the byte-code verifier.

-oss stacksize[k|m]

Sets the size of each thread's Java code stack. By default, stacksize is specified in bytes. You can specify it in kilobytes by appending the letter k or in megabytes by appending the letter m. The default value is 400 KB. You must specify at least 1000 bytes.


Output profiling information to the specified file or to the file java.prof in the current directory. The format of this profiling information is not well documented. Prior to JDK 1.1, no file can be specified; profiling information is always output to ./java.prof.

-ss stacksize[k|m]

Sets the size of each thread's native code stack. By default, stacksize is specified in bytes. You can specify it in kilobytes by appending the letter k or in megabytes by appending the letter m. The default value is 128 KB. You must specify at least 1000 bytes.


Output a trace of all bytecodes executed. java_g only.


Output a trace of all methods executed. java_g only.

-v, -verbose

Print a terminal message each time java loads a class.


Print a message whenever the garbage collector frees memory.


Run the byte-code verifier on all classes that are loaded.


Run the byte-code verifier on all classes that are loaded through a class loader. (This generally means classes that are dynamically loaded from an untrusted location.) This is the default behavior for java.


Print the version of the Java interpreter and exit.



Specifies an ordered list (colon-separated on UNIX, semicolon-separated on Windows systems) of directories and ZIP files in which java should look for class definitions. When a path is specified with this environment variable, java always implicitly appends the location of the system classes to the end of the path. If this environment variable is not specified, the default path is the current directory and the system classes. This variable is overridden by the -classpath option. See -classpath above for more information on specifying paths.

See Also

javac, jdb

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