java class version info howto

最新推荐文章于 2024-09-10 08:00:00 发布

最新推荐文章于 2024-09-10 08:00:00 发布 · 127 阅读

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#java class version info

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本文介绍了一个用于检查Java类文件版本的简单程序，通过读取类文件的魔数、主次版本号来验证文件的有效性和版本信息。

http://www.rgagnon.com/javadetails/java-0544.html
The first 4 bytes are a magic number, 0xCAFEBABe, to identify a valid class file then the next 2 bytes identify the class format version (major and minor).

Possible major/minor value :

major minor Java platform version
45 3 1.0
45 3 1.1
46 0 1.2
47 0 1.3
48 0 1.4
49 0 1.5
50 0 1.6
51 0 1.7

import java.io.*;

public class ClassVersionChecker {
public static void main(String[] args) throws IOException {
for (int i = 0; i < args.length; i++)
checkClassVersion(args[i]);
}

private static void checkClassVersion(String filename)
throws IOException
{
DataInputStream in = new DataInputStream
(new FileInputStream(filename));

int magic = in.readInt();
if(magic != 0xcafebabe) {
System.out.println(filename + " is not a valid class!");;
}
int minor = in.readUnsignedShort();
int major = in.readUnsignedShort();
System.out.println(filename + ": " + major + " . " + minor);
in.close();
}
}

> java ClassVersionChecker ClassVersionChecker.class
ClassVersionChecker.class: 49 . 0

from The Java Virtual Machine Specification

magic
The magic item supplies the magic number identifying the class file format; it has the value 0xCAFEBABE.

minor_version, major_version
The values of the minor_version and major_version items are the minor and major version numbers of this class file.Together, a major and a minor version number determine the version of the class file format. If a class file has major version number M and minor version number m, we denote the version of its class file format as M.m. Thus, class file format versions may be ordered lexicographically, for example, 1.5 < 2.0 < 2.1.

A Java virtual machine implementation can support a class file format of version v if and only if v lies in some contiguous range Mi.0 v Mj.m. Only Sun can specify what range of versions a Java virtual machine implementation conforming to a certain release level of the Java platform may support.