How can I know with which Java version a .class file is got was compiled?

Question

I'm trying to find out how I can determine the version of a .class file by looking at its binary data. I know that using javap will tell me the version - but unfortunately I don't always have a JDK installed on the machine where I need to check. I do however have access to a plain old HEX editor ...

I tried finding documentation on this from a certified source - all I was able to come up was with this: http://fredericktang.wordpress.com/2007/09/28/java-class-file-version/
Although this blog post is very nicely written - it is not really backed up by official Java documentation.

Any hint?

Edit: Never mind - sorry to bug - I found the spec: here

Answer 1

The documentation you referenced is practically a word-for-word transcription (into easier English) of the JVM class file specification.

Some of the hitoric values I keep handy

Major  Major(hex) Minor Java platform version 
45     2d          3           1.0
45     2d          3           1.1
46     2e          0           1.2
47     2f          0           1.3
48     30          0           1.4
49     31          0           1.5
50     32          0           1.6

--- Edited in response to Neil's observation that 1.0 and 1.1 use the same Major Minor numbers ---

From the class header, one cannot determine whether the class was compiled against a 1.0 or 1.1 version of Java. That said, there are a number of other techniques that can help determine if a class was compiled against 1.0 or 1.1.

First, if the class was compiled against 1.1 it might have included a 1.1 only library class. Once you find one of these in one of the classes, you can safely conclude that it's a class compiled against Java 1.1.

Note that one may search the constant pool at the beginning of the class to determine which classes the class file will need to have all of its external references fully resolved.

It can be cumbersome to search for classes individually, fortunately a lot of packages were added too. The following packages are specific to Java 1.1, that is they were not included in Java 1.0

+ java.awt.datatransfer
+ java.awt.event
+ java.beans
+ java.lang.reflect
+ java.math
+ java.rmi.dgc
+ java.rmi
+ java.rmi.registry
+ java.rmi.server
+ java.security.acl
+ java.security
+ java.security.interfaces
+ java.sql
+ java.text
+ java.util.zip

On the other side of the fence, lots of calls to the following methods suggest (but are not actual proof) that the code was written against Java 1.0, as they were deprecated in Java 1.1 due to the new awt event restructuring.

Deprecated Classes

java.io.LineNumberInputStream:
    LineNumberInputStream is deprecated. 
java.io.StringBufferInputStream:
    StringBufferInputStream is deprecated. 

Deprecated Methods

java.awt.BorderLayout:
    addLayoutComponent() is deprecated. 
java.awt.CardLayout:
    addLayoutComponent() is deprecated. 
java.awt.CheckboxGroup:
    getCurrent() is deprecated. 
java.awt.CheckboxGroup:
    setCurrent() is deprecated. 
java.awt.Choice:
    countItems() is deprecated. 
java.awt.Component:
    getPeer() is deprecated. 
java.awt.Component:
    enable() is deprecated. 
java.awt.Component:
    enable() is deprecated. 
java.awt.Component:
    disable() is deprecated. 
java.awt.Component:
    show() is deprecated. 
java.awt.Component:
    show() is deprecated. 
java.awt.Component:
    hide() is deprecated. 
java.awt.Component:
    location() is deprecated. 
java.awt.Component:
    move() is deprecated. 
java.awt.Component:
    size() is deprecated. 
java.awt.Component:
    resize() is deprecated. 
java.awt.Component:
    resize() is deprecated. 
java.awt.Component:
    bounds() is deprecated. 
java.awt.Component:
    reshape() is deprecated. 
java.awt.Component:
    preferredSize() is deprecated. 
java.awt.Component:
    minimumSize() is deprecated. 
java.awt.Component:
    layout() is deprecated. 
java.awt.Component:
    inside() is deprecated. 
java.awt.Component:
    locate() is deprecated. 
java.awt.Component:
    deliverEvent() is deprecated. 
java.awt.Component:
    postEvent() is deprecated. 
java.awt.Component:
    handleEvent() is deprecated. 
java.awt.Component:
    mouseDown() is deprecated. 
java.awt.Component:
    mouseDrag() is deprecated. 
java.awt.Component:
    mouseUp() is deprecated. 
java.awt.Component:
    mouseMove() is deprecated. 
java.awt.Component:
    mouseEnter() is deprecated. 
java.awt.Component:
    mouseExit() is deprecated. 
java.awt.Component:
    keyDown() is deprecated. 
java.awt.Component:
    keyUp() is deprecated. 
java.awt.Component:
    action() is deprecated. 
java.awt.Component:
    gotFocus() is deprecated. 
java.awt.Component:
    lostFocus() is deprecated. 
java.awt.Component:
    nextFocus() is deprecated. 
java.awt.Container:
    countComponents() is deprecated. 
java.awt.Container:
    insets() is deprecated. 
java.awt.Container:
    layout() is deprecated. 
java.awt.Container:
    preferredSize() is deprecated. 
java.awt.Container:
    minimumSize() is deprecated. 
java.awt.Container:
    deliverEvent() is deprecated. 
java.awt.Container:
    locate() is deprecated. 
java.awt.FontMetrics:
    getMaxDecent() is deprecated. 
java.awt.Frame:
    setCursor() is deprecated. 
java.awt.Frame:
    getCursorType() is deprecated. 
java.awt.Graphics:
    getClipRect() is deprecated. 
java.awt.List:
    countItems() is deprecated. 
java.awt.List:
    clear() is deprecated. 
java.awt.List:
    isSelected() is deprecated. 
java.awt.List:
    allowsMultipleSelections() is deprecated. 
java.awt.List:
    setMultipleSelections() is deprecated. 
java.awt.List:
    preferredSize() is deprecated. 
java.awt.List:
    preferredSize() is deprecated. 
java.awt.List:
    minimumSize() is deprecated. 
java.awt.List:
    minimumSize() is deprecated. 
java.awt.List:
    delItems() is deprecated. 
java.awt.Menu:
    countItems() is deprecated. 
java.awt.MenuBar:
    countMenus() is deprecated. 
java.awt.MenuComponent:
    getPeer() is deprecated. 
java.awt.MenuComponent:
    postEvent() is deprecated. 
java.awt.MenuContainer:
    postEvent() is deprecated. 
java.awt.MenuItem:
    enable() is deprecated. 
java.awt.MenuItem:
    enable() is deprecated. 
java.awt.MenuItem:
    disable() is deprecated. 
java.awt.Polygon:
    getBoundingBox() is deprecated. 
java.awt.Polygon:
    inside() is deprecated. 
java.awt.Rectangle:
    reshape() is deprecated. 
java.awt.Rectangle:
    move() is deprecated. 
java.awt.Rectangle:
    resize() is deprecated. 
java.awt.Rectangle:
    inside() is deprecated. 
java.awt.ScrollPane:
    layout() is deprecated. 
java.awt.Scrollbar:
    getVisible() is deprecated. 
java.awt.Scrollbar:
    setLineIncrement() is deprecated. 
java.awt.Scrollbar:
    getLineIncrement() is deprecated. 
java.awt.Scrollbar:
    setPageIncrement() is deprecated. 
java.awt.Scrollbar:
    getPageIncrement() is deprecated. 
java.awt.TextArea:
    insertText() is deprecated. 
java.awt.TextArea:
    appendText() is deprecated. 
java.awt.TextArea:
    replaceText() is deprecated. 
java.awt.TextArea:
    preferredSize() is deprecated. 
java.awt.TextArea:
    preferredSize() is deprecated. 
java.awt.TextArea:
    minimumSize() is deprecated. 
java.awt.TextArea:
    minimumSize() is deprecated. 
java.awt.TextField:
    setEchoCharacter() is deprecated. 
java.awt.TextField:
    preferredSize() is deprecated. 
java.awt.TextField:
    preferredSize() is deprecated. 
java.awt.TextField:
    minimumSize() is deprecated. 
java.awt.TextField:
    minimumSize() is deprecated. 
java.awt.Window:
    postEvent() is deprecated. 
java.io.ByteArrayOutputStream:
    toString() is deprecated. 
java.io.DataInputStream:
    readLine() is deprecated. 
java.io.PrintStream:
    PrintStream() is deprecated. 
java.io.PrintStream:
    PrintStream() is deprecated. 
java.io.StreamTokenizer:
    StreamTokenizer() is deprecated. 
java.lang.Character:
    isJavaLetter() is deprecated. 
java.lang.Character:
    isJavaLetterOrDigit() is deprecated. 
java.lang.Character:
    isSpace() is deprecated. 
java.lang.ClassLoader:
    defineClass() is deprecated. 
java.lang.Runtime:
    getLocalizedInputStream() is deprecated. 
java.lang.Runtime:
    getLocalizedOutputStream() is deprecated. 
java.lang.String:
    String() is deprecated. 
java.lang.String:
    String() is deprecated. 
java.lang.String:
    getBytes() is deprecated. 
java.lang.System:
    getenv() is deprecated. 
java.net.Socket:
    Socket() is deprecated. 
java.net.Socket:
    Socket() is deprecated. 
java.util.Date:
    Date() is deprecated. 
java.util.Date:
    Date() is deprecated. 
java.util.Date:
    Date() is deprecated. 
java.util.Date:
    Date() is deprecated. 
java.util.Date:
    UTC() is deprecated. 
java.util.Date:
    parse() is deprecated. 
java.util.Date:
    getYear() is deprecated. 
java.util.Date:
    setYear() is deprecated. 
java.util.Date:
    getMonth() is deprecated. 
java.util.Date:
    setMonth() is deprecated. 
java.util.Date:
    getDate() is deprecated. 
java.util.Date:
    setDate() is deprecated. 
java.util.Date:
    getDay() is deprecated. 
java.util.Date:
    getHours() is deprecated. 
java.util.Date:
    setHours() is deprecated. 
java.util.Date:
    getMinutes() is deprecated. 
java.util.Date:
    setMinutes() is deprecated. 
java.util.Date:
    getSeconds() is deprecated. 
java.util.Date:
    setSeconds() is deprecated. 
java.util.Date:
    toLocaleString() is deprecated. 
java.util.Date:
    toGMTString() is deprecated. 
java.util.Date:
    getTimezoneOffset() is deprecated. 

It is possible to prove that Java code is version 1.1, but only in some cases (if the code uses newly added 1.1 features). It is generally impossible to prove that Java code was 1.0, but the lack of any use of Java 1.1 features, and heavy use of Java's deprecated features hints to classes targeted for the Java 1.0 environment.

I am unaware of any in-class tagging which might be able to differentiate the two (differences in line number tables, etc.), mostly because I haven't had the need to dig through 1.0 and 1.1 classes in a hex editor.

Answer 2

The JVM specification is about as authoritative as you're going to get. Chapter 4 is probably what you're after.