Using the java command (with examples)

1: Executing a java .class file

java classname

Motivation: This use case is used to execute a Java program that contains a main method. It is the most common and straightforward usage of the java command.

Explanation: The java command is followed by the name of the class file to be executed. The class file must contain a main method, which is the entry point of the Java program.

Example Output:

Hello, World!

If the classname class file contains a main method that prints “Hello, World!”, executing java classname will display “Hello, World!” on the console.

2: Executing a java program with additional classes

java -classpath path/to/classes1:path/to/classes2:. classname

Motivation: This use case is used when a Java program depends on external or user-defined classes that are not in the same directory as the class file being executed.

Explanation: The -classpath (or -cp) option allows specifying additional directories or JAR files where the Java runtime should look for classes. The value of the -classpath option is a colon-separated list of directories/JAR files. The . (dot) represents the current directory.

Example Output:

Hello, John!

Assume that the class Greeting is in the com/example/util package, located in the directory path/to/classes1. When executing java -classpath path/to/classes1:. com.example.Main, the Java program will use the Greeting class from path/to/classes1 and display “Hello, John!” on the console.

3: Executing a .jar program

java -jar filename.jar

Motivation: This use case is used to execute Java programs that are bundled as executable JAR files. JAR files provide a way to package all the necessary classes and resources into a single file.

Explanation: The -jar option specifies that the Java program should be executed from a JAR file. The filename.jar is the name of the JAR file to be executed.

Example Output:

Hello, World!

If filename.jar is a JAR file that contains a Main class with a main method that prints “Hello, World!”, executing java -jar filename.jar will display “Hello, World!” on the console.

4: Executing a .jar program with debug

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

Motivation: This use case is used when we need to enable remote debugging for a Java program in a JAR file. It allows us to connect a debugger to the running Java process.

Explanation: The -agentlib:jdwp option enables Java Debug Wire Protocol (JDWP) for debugging. The transport=dt_socket indicates that the debugger will connect through a socket. The server=y tells the JVM to listen for a debugger connection. The suspend=y pauses the JVM until a debugger connects. The address=*:5005 specifies the hostname and port (5005 in this case) on which the JVM should listen for debugger connections.

Example Output:

Listening for transport dt_socket at address: 5005

Executing java -agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=*:5005 -jar filename.jar will pause the JVM and provide the output indicating that it is waiting for a debugger to connect on port 5005.

5: Displaying JDK, JRE, and HotSpot versions

java -version

Motivation: This use case is used to quickly check the version of the installed JDK (Java Development Kit), JRE (Java Runtime Environment), and HotSpot (Java Virtual Machine).

Explanation: The -version option is a built-in option of the java command that displays the version information of the installed Java runtime environment.

Example Output:

openjdk version "1.8.0_292"
OpenJDK Runtime Environment (AdoptOpenJDK)(build 1.8.0_292-b10)
OpenJDK 64-Bit Server VM (AdoptOpenJDK)(build 25.292-b10, mixed mode)

Executing java -version will display the version information of the currently installed Java runtime environment.

6: Displaying usage information for the java command

java -help

Motivation: This use case is used to get detailed information about the usage of the java command, including all available options and arguments.

Explanation: The -help option is a built-in option of the java command that displays the usage information.

Example Output:

Usage: java [options] <mainclass> [args...]

Commands:
...
  -h, --help                 print this help message
...

Executing java -help will display detailed usage information for the java command, including available options and arguments.