查看Java class文件版本的方法

查看Java class文件版本的方法
查看的方法：
1.先装一个DJ java Decompiler 这个工具网上到处都是，主要就是反编译
Java class文件。

2.在DJ Java Decompiler的菜单中View->class version info，就会弹出对话框如：
Major Version: 49
Minor Version: 0
参照下面的引用，可知class由jdk1.5编译而来。

jackpk已经亲测，以上方法非常实用。

关于JDK版本，参见：
《unsupported major.minor version 解决方法》
（/jackpk/article/details/19755119）
引用
/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
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.。