Java API找出JDK版本的类文件是为其编译的？

Question

是否有任何Java API来查找为其编译的类文件的JDK版本？当然有javap工具可以找到这里提到的主要版本.但是,我想以编程方式执行此操作,以便我可以警告用户为适当的JDK编译它

Answer 1

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();
    }
}

可能的值是:

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           5
50       0           6
51       0           7
52       0           8
53       0           9
54       0           10
55       0           11

Answer 2

basszero的方法可以通过UNIX命令行和"od(1)"命令完成:

% od -x HelloWorldJava.class |head -2
0000000 feca beba 0000 3100 dc00 0007 0102 2a00
0000020 6f63 2f6d 6e65 6564 6163 642f 6d65 2f6f

"feca beba"是神奇的数字."0000 3100"是0x31,表示J2SE 5.0.

Answer 3

这将获取类文件的内容:

MysteryClass.class.getResourceAsStream("MysteryClass.class")

然后查看字节5-8以获取次要版本和主要版本.可在此处找到这些数字与JDK版本之间的映射.

Answer 4

Apache BCEL提供此API:

JavaClass c = Repository.lookupClass("com.x.MyClass")
c.getMinor();
c.getMajor();

Answer 5

只需直接读取类文件即可.找出版本非常容易.查看规范和维基,然后尝试代码.包装此代码并制作更有用/漂亮是留作练习.或者,您可以使用BCEL,ASM或JavaAssist等库

import java.io.DataInputStream;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;

public class ClassVersionTest
{
    private static final int JAVA_CLASS_MAGIC = 0xCAFEBABE;

    public static void main(String[] args)
    {
        try
        {
            DataInputStream dis = new DataInputStream(new FileInputStream("Test.class"));
            int magic = dis.readInt();
            if(magic == JAVA_CLASS_MAGIC)
            {
                int minorVersion = dis.readUnsignedShort();
                int majorVersion = dis.readUnsignedShort();

                /**
                 * majorVersion is ...
                 * J2SE 6.0 = 50 (0x32 hex),
                 * J2SE 5.0 = 49 (0x31 hex),
                 * JDK 1.4 = 48 (0x30 hex),
                 * JDK 1.3 = 47 (0x2F hex),
                 * JDK 1.2 = 46 (0x2E hex),
                 * JDK 1.1 = 45 (0x2D hex).
                 */

                System.out.println("ClassVersionTest.main() " + majorVersion + "." + minorVersion);
            }
            else
            {
                // not a class file
            }
        }
        catch (FileNotFoundException e)
        {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        catch (IOException e)
        {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}

Answer 6

在Linux上，您可以file在类文件上使用该命令，对于我的用例，我发现它最简单。例如：

$ file Foo.class
Foo.class: compiled Java class data, version 50.0 (Java 1.6)