Week 156 — What is the javap tool and what is it used for?
Question of the Week #156
What is the javap tool and what is it used for?
K is a program that comes with the JDK which allows viewing the content of class files in a text format.
For example, consider the following class (located in ):
When we compile that (), a file named is created.
When we view that class file with , it shows a bit of information about the content of the class file including its structure:
If we also want to see members, we can add the (or flag):
Similarly, the flag results in only members:
For example, consider the following class (located in ):
When we compile that (), a file named is created.
When we view that class file with , it shows a bit of information about the content of the class file including its structure:
If we also want to see members, we can add the (or flag):
Similarly, the flag results in only members:
KSometimes, don't just want to view the structure of a class but also its bytecode instructions. For that, the option allows to disassemble the bytecode showing textual representation of the bytecode instructions:
Here, we can see hat the (non-) method pushes the receiver () object () as well as the constant value () to the (Java) stack and stores the value to the field (). These instructions correspond to the statement.
Then, it pushes both the content of the field (), the receiver () object () and content of the variable () to the stack and calls . That corresponds to the statement in the method.
Finally, it ends the method with a instruction.
Here, we can see hat the (non-) method pushes the receiver () object () as well as the constant value () to the (Java) stack and stores the value to the field (). These instructions correspond to the statement.
Then, it pushes both the content of the field (), the receiver () object () and content of the variable () to the stack and calls . That corresponds to the statement in the method.
Finally, it ends the method with a instruction.
KIf we want to confirm which lines these statements correspond to, we can use the argument which also prints the line number table:
Here, we can see that the bytecode corresponding to line 4 starts at the first , the bytecode for line 5 starts at the instruction and the bytecode for line 6 is located at the instruction. This matches the class we wrote before:
Here, we can see that the bytecode corresponding to line 4 starts at the first , the bytecode for line 5 starts at the instruction and the bytecode for line 6 is located at the instruction. This matches the class we wrote before:
KIn addition the structure and the content of the methods, class files contain quite a bit of additional information which can be viewed with the option (e.g. using ).
This includes things like the major and minor version of the class file format (e.g. 69 for JDK 25), the constant pool and additional information about methods and fields (e.g. flags).
This includes things like the major and minor version of the class file format (e.g. 69 for JDK 25), the constant pool and additional information about methods and fields (e.g. flags).
javap_v.txt2.72KB
KThe tool also includes a script which can be opened using the command or by adding it to the command (). This script includes a method that allows using tool on classes which are available to . It can be invoked like any other method and takes the object representing the class to view as an argument.
jshell_output.txt2.76KB
KJavap is a Java Disassembler tool that comes with the JDK. This tool is used to display the bytecode inside .class files in a readable format.
KI think my answer is right
Kjavap is a disassembler for Java bytecode. Input to the tool are compiled Java class files (.class). Output is JVM assembly which is a textual representation of the bytecode bytes in the .class file. It’s not a decompiler - it won’t generate Java source code, but instead JVM assembly instructions.
Simple example:
javap Foo.class
Without options javap displays all the public, protected, and package-private fields and methods of the input .class file(s). There are various flags you can include to specify various output options.
javap -private Foo.class
will also include private fields and methods.
javap -c Foo.class
will show disassembled bytecode (JVM assembly) for the methods it displays.
other flags let you display type signatures (-s), show stack size and local variable tables (-verbose), show constants table (static final variables) (-constants).
It’s a useful debugging tool and teaching tool that lets you observe how javac compiles your source code into bytecode.
Simple example:
javap Foo.class
Without options javap displays all the public, protected, and package-private fields and methods of the input .class file(s). There are various flags you can include to specify various output options.
javap -private Foo.class
will also include private fields and methods.
javap -c Foo.class
will show disassembled bytecode (JVM assembly) for the methods it displays.
other flags let you display type signatures (-s), show stack size and local variable tables (-verbose), show constants table (static final variables) (-constants).
It’s a useful debugging tool and teaching tool that lets you observe how javac compiles your source code into bytecode.