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Common Commands in JDK

Posted Jan 25, 2024
In (CS) Learning Note
7 min read

Java Development Kit (JDK) offers commands that are essential for Java development, each serving a specific purpose. Understanding these commands and their parameters is crucial for any Java developer.

Contents

javac (Java compiler)

Purpose: javac is the Java compiler command. It’s used to compile Java source files into bytecode (.class files).

Some Parameters:

  • -classpath or -cp: Set the classpath.
  • -d: Specify the destination directory for generated class files.

Example:

Compile a Java file named Example.java:

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javac Example.java

Compile multiple Java files in a specific directory:

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javac -d ./bin ./src/*.java

java (Java launcher)

Purpose: The java command is used to launch a Java application. It starts the Java runtime environment, loads the specified class, and calls its main method.

Some Parameters:

  • -classpath or -cp: Set the classpath.
  • -Dproperty=value: Set a system property.
  • -Xmx: Maximum size of the memory allocation pool.
  • -Xms: Initial size of the memory allocation pool.

Example:

Run a class named Example:

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java -cp ./bin Example

Run with increased heap size:

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java -Xmx512m -Xms256m -cp ./bin Example

jar (Creat and manage JAR files)

Purpose: Used for creating and managing JAR (Java Archive) files, which bundle multiple Java class files and associated metadata into a single file.

Some Parameters:

  • c: Creates a new JAR file.
  • f: Specifies the JAR file name.
  • v: Generates verbose output.
  • x: Extracts files from a JAR file.

Example:

Create a JAR file:

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jar cfv example.jar -C ./bin .

Extract a JAR file:

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jar xvf example.jar

javap (Java Class File Disassembler)

Purpose: Disassembles class files.

Some Parameters:

  • -c: Disassembles the bytecode.
  • <classfile>: The name of the class file to disassemble.

Example:

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javap -c MyClass.class

Expected Response:

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Compiled from "MyClass.java"
public class MyClass extends java.lang.Object{
public MyClass();
Code:
0: aload_0
1: invokespecial #1                  // Method java/lang/Object."<init>":()V
4: return

    public void myMethod();
    Code:
       0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
       3: ldc           #3                  // String Hello, World!
       5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       8: return
}

Displays disassembled bytecode of the MyClass class file.

javadoc (Generates HTML documentation)

Purpose: Generates HTML documentation from Java source code with special documentation comments.

Some Parameters:

  • -d: Destination directory for output.
  • -sourcepath: Source files path.

Example:

Generate documentation for all Java files in a directory:

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javadoc -d ./docs -sourcepath ./src

jconsole (Monitoring tool)

Purpose: jconsole is a monitoring tool that provides information about the performance and resource consumption of running Java applications.

Example:

Launch JConsole:

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jconsole

jps (Java Virtual Machine Process Status Tool)

Purpose: JVM Process Status Tool, displays all HotSpot virtual machine processes on a given system.

Example:

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jps -l

Expected Response:

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2234 sun.tools.jps.Jps
4691 my.java.Application

Lists running Java processes with their process ids and the main class name.

jstat (JVM Statistics Monitoring Tool)

Purpose: Monitors JVM statistics. Tools used to monitor the virtual machine a variety of operational status information. Including class loading, memory, rubbish collection, JIT compilation and other operational data.

Some Parameters:

  • -gc: Monitors garbage collection statistics.
  • <vmid>: The process id of the JVM.
  • <interval>: The time between data samples in milliseconds.
  • <count>: The number of samples to take before terminating.

Example:

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jstat -gc 4691 250 4

Expected Response:

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S0C    S1C    S0U    S1U      EC       EU        OC         OU       MC     MU    CCSC   CCSU   YGC     YGCT    FGC    FGCT     GCT   
46080.0 46080.0 0.0    0.0    184320.0  11614.4  368640.0   32252.8  4480.0 4345.7 512.0  498.2      5    0.305   2      0.273    0.578

Displays garbage collection statistics for the JVM with process id 4691.

  1. S0C: capacity of the first survivor in the young generation (KB)
  2. S1C: capacity of the second survivor in the young generation (KB)
  3. S0U: currently used space of the first survivor in the young generation (KB)
  4. S1U: currently used space of the second survivor in the young generation (KB)
  5. EC: capacity of Eden in the young generation (KB)
  6. EU: currently used space of Eden in the young generation (KB)
  7. OC: Capacity of the Old generation (KB)
  8. OU: Currently used space in Old generation (KB)
  9. PC: Capacity of Perm (Persistent Generation) (KB)
  10. PU: Perm Used Space (KB)
  11. YGC: number of gc’s in the young generation from application startup to sampling
  12. YGCT: time from application startup to gc in the young generation at the time of sampling (s)
  13. FGC: number of gc in old generation (full gc) from application startup to sampling time
  14. FGCT: time(s) spent in gc from application startup to gc in old generation (full gc) at sample time
  15. GCT: total time(s) used for gc from application startup to sampling time

jinfo (Configuration Info for Java)

Purpose: Shows configuration info for a Java process.

Some Parameters:

  • <vmid>: The process id of the JVM.

Example:

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jinfo 4691

Expected Response:

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Attaching to process ID 4691, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.131-b11
Java System Properties:

java.runtime.name = Java(TM) SE Runtime Environment
java.vm.version = 25.131-b11
java.vm.vendor = Oracle Corporation

Lists configuration information for the JVM with process id 4691.

jmap (Memory Map for Java)

Purpose: Prints memory-related information of a Java process.

Some Parameters:

  • -heap: Prints heap summary information.
  • <vmid>: The process id of the JVM.

Example:

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jmap -heap 4691

Expected Response:

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Attaching to process ID 4691, please wait...
Debugger attached successfully.

Heap Configuration:
MinHeapFreeRatio = 40
MaxHeapFreeRatio = 70
MaxHeapSize      = 2097152000 (2000.0MB)
NewSize          = 1310720 (1.25MB)
MaxNewSize       = 17592186044415 MB
OldSize          = 5439488 (5.1875MB)
NewRatio         = 2
SurvivorRatio    = 8
MetaspaceSize    = 21807104 (20.796875MB)
CompressedClassSpaceSize = 1073741824 (1024.0MB)
MaxMetaspaceSize = 17592186044415 MB
G1HeapRegionSize = 0 (0.0MB)

Displays heap configuration and utilization for the JVM process.

jhat (JVM Heap Analysis Tool)

Purpose: Analyzes heap dump files.It is used in conjunction with jmap to analyse the heapdump(.hprof) generated by jmap.

Some Parameters:

  • <heapdumpfile>: The filename of the heap dump.

Example:

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jhat -J-Xmx4G heapdump.hprof

Expected Response:

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Reading from heapdump.hprof...
Dump file created Thu Jan 07 10:32:18 PST 2021
Snapshot read, resolving...
Resolving 9876543 objects...
Chasing references, expect 12345 dots....................................
Snapshot resolved.
Started HTTP server on port 7000
Server is ready.

Analyzes the heap dump file and starts a web server for analysis.

jstack (Stack Trace for Java)

Purpose: Prints stack traces of Java threads. jstack is used to generate a snapshot of the threads in the java virtual machine at the current moment. A thread snapshot is a collection of method stacks for each thread currently executing within the java virtual machine. The main purpose of generating thread snapshots is to locate the reasons for threads to appear a long time to stop , such as thread deadlocks, dead loops, requests for external resources resulting in a long time to wait and so on.

Some Parameters:

  • <vmid>: The process id of the JVM.

Example:

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jstack 4691

Expected Response:

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Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.131-b11 mixed mode):

"Attach Listener" #14 daemon prio=9 os_prio=0 tid=0x00007f13c4001000 nid=0x1234 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE

"Finalizer" #3 daemon prio=8 os_prio=0 tid=0x00007f13c000a800 nid=0x1107 in Object.wait() [0x00007f13b4ffe000]
java.lang.Thread.State: WAITING (on object monitor)

Provides a snapshot of all threads’ stack traces in the JVM.

(CS) Learning Note, Java
software engineering Java JVM
This post is licensed under CC BY 4.0 by the author.
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