打印class文件的Java编译器内部的版本号

import java.io.DataInputStream;

import java.io.File;

import java.io.FileInputStream;

import java.io.IOException;

public class ClassVersionChecker {

    public static void main(String[] args) throws IOException {

        System.out.println(System.getProperty("java.version"));

        String canonicalName = ClassVersionChecker.class.getCanonicalName();

        String classPathSeparate = "/";

        String extension = ".class";

        String path = ClassVersionChecker.class.getResource(classPathSeparate + canonicalName.replace(".", classPathSeparate) + extension).getPath();

        File file = new File(path);

        checkClassVersion(file);

    }

    private static void checkClassVersion(File classFile)

            throws IOException {

        DataInputStream in = new DataInputStream

                (new FileInputStream(classFile));

        int magic = in.readInt();

        if (magic != 0xcafebabe) {

            System.out.println(classFile.getName() + " is not a valid class!");

        }

        int minor = in.readUnsignedShort();

        int major = in.readUnsignedShort();

        System.out.println(classFile.getName() + ": " + major + " . " + minor);

        in.close();

    }

} 

JDK1.8:

1.8.0_65

ClassVersionChecker.class: 52 . 0

JDK1.6：

1.6.0_45

ClassVersionChecker.class: 50 . 0