java

Execute a Java .class file that contains a main method by using just the class name

$ java [classname]

$ java -classpath [path/to/classes1]:[path/to/classes2]:. [classname]

$ java -jar [filename.jar]

$ java -agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=*:5005 -jar [filename.jar]

$ java -version

$ java -help

java [options] mainclass [args]
java [options] --jar jarfile [args]
java [options] -m module[/mainclass] [args]

-version
    Print product version and exit

-v or --version
    Print brief version (Java 9+)

-cp <path> or -classpath <path>
    Set classpath for classes/jars

--jar <jar>
    Execute named jar file

-jar <jar>
    Legacy jar execution (use --jar)

-m or --module <module>[/<mainclass>]
    Run module (Java 9+)

-p or --module-path <path>
    Module search path

-D<name>=<value>
    Set system property

-Xmx<size>
    Max heap size (e.g., -Xmx1g)

-Xms<size>
    Initial heap size

-XX:+UseG1GC
    Enable G1 garbage collector (default Java 9+)

-verbose:class
    Verbose class loading

-verbose:gc
    Verbose garbage collection

--enable-preview
    Enable preview features

-agentlib:<libname>
    Load native agent library

The java command starts a Java Virtual Machine (JVM) to execute Java programs, applets, or JAR files. Part of the JDK/JRE, it loads bytecode, performs verification, and runs the main class method. Supports class files, executable JARs, and modules (Java 9+).

Common uses include running console apps, servers (e.g., Tomcat), or GUI tools. Options tune performance: memory allocation (-Xmx/-Xms), garbage collection (-XX flags), debugging (-agentlib), and security. Classpath management via -cp allows specifying libraries.

The JVM handles platform independence via Just-In-Time (JIT) compilation, converting bytecode to native code. Multiple implementations exist: OpenJDK (default on many Linux distros), Oracle JDK. Environment variables like JAVA_HOME and CLASSPATH influence behavior.

For production, set heap sizes to avoid OutOfMemoryError. Modular apps use -p/-m for better encapsulation. Verbose modes (-verbose:class/gc) aid troubleshooting. Exits with codes: 0 success, non-zero errors.

Unlimited options exist (-X, -XX); consult java --help or man page. Heap too small causes OutOfMemoryError. Untrusted code risks security; use sandboxing. Multiple JVMs may conflict; specify full path.

JAVA_HOME: JDK install dir.
CLASSPATH: Default class path.
_JAVA_OPTIONS: Options passed to JVM.

Use update-alternatives --config java on Debian/Ubuntu to switch versions.

Developed by Sun Microsystems (1995) for JDK 1.0. Evolved with HotSpot JVM (1999), generics (1.5), modules (9, 2017). OpenJDK project (2006) now dominant; Oracle JDK commercial. Linux support via packages like openjdk-17-jdk.

javac(1), jar(1), javadoc(1), jdb(1), keytool(1), jps(1)