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