How to use the command 'javap' (with examples)

The javap command in Java is used to disassemble one or more class files and list them. It provides detailed information about the fields, methods, and constructors present within a class file. This can be useful for understanding the underlying structure and implementation of a Java class.

Use case 1: Disassemble and list a .class file

Code:

javap path/to/file.class

Motivation: You may want to disassemble and inspect the contents of a specific .class file to understand its structure and implementation. This can be helpful for debugging or gaining deeper insights into how a particular class works.

Explanation: The javap command is followed by the path to the .class file you want to disassemble. This path can be either absolute or relative to the current directory.

Example output:

Compiled from "ClassName.java"
public class ClassName {
  public ClassName();
  public static void main(java.lang.String[]);
}

The output displays the class name, constructors, and methods present within the specified .class file.

Use case 2: Disassemble and list multiple .class files

Code:

javap path/to/file1.class path/to/file2.class ...

Motivation: You may have multiple .class files that you want to disassemble and analyze together. This can be useful when you want to compare the structures and implementations of different classes or identify relationships between them.

Explanation: The javap command is followed by the paths to the .class files you want to disassemble. You can specify multiple file paths separated by spaces.

Example output:

Compiled from "Class1.java"
public class Class1 {
  public Class1();
  public static void method1();
}

Compiled from "Class2.java"
public class Class2 {
  public Class2();
  public static void method2();
}

The output displays the class name, constructors, and methods for each of the specified .class files.

Use case 3: Disassemble and list a built-in class file

Code:

javap java.package.class

Motivation: You may want to analyze the internal structure and implementation of a built-in Java class, such as java.util.ArrayList or java.lang.String, to gain insights into their behavior and use.

Explanation: The javap command is followed by the package and class name of the built-in Java class you want to disassemble. The package and class name are separated by a dot (.).

Example output:

Compiled from "ArrayList.java"
public class java.util.ArrayList<E> extends java.util.AbstractList<E> 
  implements java.util.List<E>, java.util.RandomAccess, java.lang.Cloneable, java.io.Serializable {
  public java.util.ArrayList();
  public java.util.ArrayList(int);
  public java.util.ArrayList(java.util.Collection<? extends E>);
  public void trimToSize();
  // ...
}

The output displays the class name, inheritance hierarchy, constructors, and methods for the specified built-in Java class.

Use case 4: Display help

Code:

javap -help

Motivation: If you want to quickly access the help documentation for the javap command, you can use this option.

Explanation: The javap command is followed by the -help option to display the help information for the command.

Example output:

Usage: javap <options> <classes>
where <options> include:
    -help                  print this help message
    -version               print version information
    -v                     show additional information
    ...

The output provides a summary of the available options and their descriptions.

Use case 5: Display version

Code:

javap -version

Motivation: If you want to know the version information for the javap command, you can use this option.

Explanation: The javap command is followed by the -version option to display the version information of the command.

Example output:

javap 16

The output shows the version number of the javap command.

Conclusion:

The javap command is a powerful tool for disassembling Java class files and analyzing their contents. Whether you want to inspect a single class file, compare multiple class files, analyze built-in classes, or access help and version information, the javap command provides the necessary functionality. Understanding the structure and implementation of Java classes can help in debugging, performance tuning, and gaining deeper insights into the behavior of Java applications.