java的class文件的前4个字节叫做magic number,

用来识别是否为java的class文件.

package pkg;

import java.io.File;

import java.io.FileInputStream;

import java.io.FileNotFoundException;

import java.io.IOException;

/**

* @author qefee.com

*

*/

public class MagicNumber {

public static void main(String[] args) {

String pathname = "./bin/pkg/MagicNumber.class";

File file = new File(pathname);

FileInputStream fis = null;

try {

fis = new FileInputStream(file);

int magicNumber = 0;

magicNumber = fis.read();

magicNumber = (magicNumber << 8) ^ fis.read();

magicNumber = (magicNumber << 8) ^ fis.read();

magicNumber = (magicNumber << 8) ^ fis.read();

String magicNumberString = Integer.toHexString(magicNumber);

System.out.println(magicNumber);

System.out.println(magicNumberString);

} catch (FileNotFoundException e) {

e.printStackTrace();

} catch (IOException e) {

e.printStackTrace();

} finally {

if (fis != null) {

try {

fis.close();

} catch (IOException e) {

e.printStackTrace();

}

}

}

}

}

或者

package pkg;

import java.io.File;

import java.io.FileInputStream;

import java.io.FileNotFoundException;

import java.io.IOException;

/**

* @author qefee.com

*

*/

public class MagicNumber {

public static void main(String[] args) {

String pathname = "./bin/pkg/MagicNumber.class";

File file = new File(pathname);

FileInputStream fis = null;

try {

fis = new FileInputStream(file);

System.out.print(Integer.toHexString(fis.read()));

System.out.print(Integer.toHexString(fis.read()));

System.out.print(Integer.toHexString(fis.read()));

System.out.print(Integer.toHexString(fis.read()));

} catch (FileNotFoundException e) {

e.printStackTrace();

} catch (IOException e) {

e.printStackTrace();

} finally {

if (fis != null) {

try {

fis.close();

} catch (IOException e) {

e.printStackTrace();

}

}

}

}

}

输出

cafebabe