Java CLI JDK command line

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Start here

java Hello.java

Compile and run one file

javac -d out src/*.java

Compile to a folder

java -cp out Main

Run a class

jar --create --file app.jar --main-class Main -C out
                    .

Build a runnable JAR

java -jar app.jar

Run a JAR

java -Xmx2g -jar app.jar

Cap the heap

jshell

Try code interactively

jcmd

List running JVMs

Concepts

Main.java source
Main.class bytecode
app.jar archive
JVM running app

Run

java -version

Print version and exit

Goes to stderr; --version prints to stdout.

java <File>.java [args]

Compile in memory and run

Runs the first class in the file; it needs a main method.

java -cp <path> <MainClass> [args]

Run a class

java -jar <file.jar> [args]

Run a JAR's main class

-cp is ignored here; use the manifest Class-Path.

java -Dkey=value ...

Set a system property

Read it with System.getProperty("key").

java -ea ...

Enable assertions

java -m <module>/<MainClass>

Run a module

java --enable-preview ...

Allow preview features

java @<argfile>

Read options from a file

java --help

Launcher options

java -X

Extra -X options

Class path & modules

-cp "lib/*:out"

Add all JARs in lib, plus out

Quote it so the shell does not expand the star.

--module-path <dir> / -p <dir>

Where application modules live

--add-modules <mod>[,<mod>]

Add root modules

--add-opens <mod>/<pkg>=ALL-UNNAMED

Open a package to reflection

Fixes InaccessibleObjectException on JDK 16+.

--add-exports <mod>/<pkg>=ALL-UNNAMED

Export a package

java --list-modules

List observable modules

java --describe-module <mod>

Describe one module

Memory & JVM flags

-Xmx<size>

Maximum heap

Suffixes k, m, g: -Xmx512m, -Xmx2g.

-Xms<size>

Initial heap

-Xss<size>

Thread stack size

Raise it on StackOverflowError in deep recursion.

-XX:MaxRAMPercentage=<n>

Heap as share of RAM

Handy in containers instead of a fixed -Xmx.

-XX:+HeapDumpOnOutOfMemoryError

Write a heap dump on OOM

-XX:HeapDumpPath=<file>

Where the dump goes

-verbose:gc

Log garbage collections

java -XX:+PrintFlagsFinal -version

Show all flag values

Compile (javac)

javac <File>.java

Compile one file

javac -d <dir> <files>

Put class files in dir

The directory is created if it is missing.

javac -cp <path> <files>

Compile against libraries

javac --release <n> <files>

Target a Java version

Also checks against that release's API.

javac -Xlint:all <files>

Enable all warnings

javac -encoding UTF-8 <files>

Source file encoding

javac -sourcepath <dir> <File>.java

Find other sources

javac -parameters <files>

Keep parameter names

javac -Werror <files>

Fail on warnings

javac @<argfile>

Options and files from a file

Package (jar)

jar --create --file <f.jar> -C <dir> .

Pack a directory

jar --create --file <f.jar> --main-class
                    <Class> -C <dir> .

Runnable JAR

Writes Main-Class into META-INF/MANIFEST.MF.

jar -tf <f.jar>

List contents

jar -xf <f.jar> [<entry>]

Extract all or one entry

jar --update --file <f.jar> -C <dir>
                    <file>

Add or replace a file

jar --create --file <f.jar> --manifest <file>
                    -C <dir> .

Use your own manifest

jar --describe-module --file <f.jar>

Show module descriptor

jshell

jshell

Start the REPL

Leave with /exit.

jshell --class-path <path>

Start with libraries

/vars

Show variables

/methods

Show methods

/imports

Show active imports

/list

Show snippets with IDs

/save <file>

Save snippets to a file

/open <file>

Load a script

/reset

Discard all snippets

/help

List commands

Diagnose

jcmd

List running JVMs

jps -l

PIDs with main class or JAR

jcmd <pid> Thread.print

Thread dump

Add -l for lock information.

jstack <pid>

Thread dump (older tool)

jcmd <pid> GC.heap_info

Heap usage summary

jcmd <pid> GC.class_histogram

Objects per class

jcmd <pid> GC.heap_dump <file>

Write a heap dump

Slow and large; use -gz=1 to compress.

jcmd <pid> VM.flags

Flags of the running JVM

jcmd <pid> VM.system_properties

System properties

jcmd <pid> VM.command_line

How it was started

jcmd <pid> JFR.start duration=60s
                    filename=<f.jfr>

Record a flight recording

jcmd <pid> help

Commands this JVM knows

Environment

JAVA_HOME

JDK install directory

JAVA_TOOL_OPTIONS

Options for every JVM

Picked up by all JVMs, including build tools; the JVM prints a notice.

JDK_JAVA_OPTIONS

Options for the java launcher

Only the java command reads it; -jar and -h are not allowed in it.

CLASSPATH

Default class path

Ignored when -cp is given.

Recipes

Build and run a small app

javac -d out $(find src -name '*.java')jar --create --file app.jar --main-class com.example.Main -C out .java -jar app.jar

Run with libraries

javac -cp "lib/*" -d out src/Main.javajava -cp "out:lib/*" Main

Diagnose a stuck app

jps -ljcmd <pid> Thread.print -l > threads.txtjcmd <pid> GC.heap_info

Capture memory failures

java -Xmx512m \  -XX:+HeapDumpOnOutOfMemoryError \  -XX:HeapDumpPath=/tmp/heap.hprof \  -jar app.jar