鉴定文件是否为java class文件，是否是可执行文件

1、是否是Java程序？

是。十六进制文本编辑器打开，前四个字节为0xCAFEBABE，这是Java文件的标志

2、是否可执行？

       不可执行。

 

       测试环境：

       Sun j2sdk1.4.2_08

 

       执行：

java CheckFileB

 

       执行结果如下：出现异常ClassFormatError

 

分析原因：

1、  类加载时class文件格式不正确等原因导致类不可加载

2、  程序执行过程中自身抛出此异常，故意抛出异常，或调用别的类时出现异常。

 

如果为原因1，此程序尚未执行，没有进一步危害。

如果为原因2，程序可能已经执行，可能会有其它危害。

 

进一步分析

       修改j2sdk1.4.2_08中的ClassLoader部分源码，对类装载过程进行跟踪，修改如下：

      

protected final Class defineClass(String name, byte[] b, int off, int len,                                       ProtectionDomain protectionDomain) throws         ClassFormatError {         System.out.println("");       System.out.println("================== Define Class " + name +                          " ====================");         Class c = null;         check();       if ( (name != null) && name.startsWith("java.")) {         System.out.println("### Exception : Class name is start with java");         throw new SecurityException("Prohibited package name: " +                                     name.substring(0, name.lastIndexOf('.')));       }       if (protectionDomain == null) {         protectionDomain = getDefaultDomain();       }         if (name != null) {         checkCerts(name, protectionDomain.getCodeSource());       }         try {         if (!checkName(name, false)) {           System.out.println("### Error : No Class Def Found, Illegal name " +                              name);           throw new NoClassDefFoundError("Illegal name: " + name);         }           c = defineClass0(name, b, off, len, protectionDomain);         System.out.println("### Get class define!! ");       }       catch (ClassFormatError cfe) {         // Class format error - try to transform the bytecode and         // define the class again         //         System.out.println("### Catch ClassFormatError, Try other transformers");         Object[] transformers = ClassFileTransformer.getTransformers();           if (transformers == null || transformers.length == 0)           System.out.println("### No other transformers!");           for (int i = 0; transformers != null && i < transformers.length; i++) {           try {             // Transform byte code using transformer             byte[] tb = ( (ClassFileTransformer) transformers[i]).transform(b,                 off, len);             c = defineClass0(name, tb, 0, tb.length, protectionDomain);             break;           }           catch (ClassFormatError cfe2) {             // If ClassFormatError occurs, try next transformer             System.out.println("### Total " + transformers.length +                                " Transformers, the " + i +                                " failed to transform.");             if (i == transformers.length)               System.out.println(                   "### After all transformer , still cannot get this Class");             }         }           // Rethrow original ClassFormatError if unable to transform         // bytecode to well-formed         //         if (c == null) {           System.out.println("### Error : Class Format Error! ###");           throw cfe;         }         else           System.out.println("### Get Class :" + c.getName() + " " + c.toString());       }         if (protectionDomain.getCodeSource() != null) {         java.security.cert.Certificate certs[] =             protectionDomain.getCodeSource().getCertificates();         if (certs != null)           setSigners(c, certs);       }         System.out.println("### return from defineClass() value: " + c.toString());         return c;     }

 

protected final Class findSystemClass(String name)       throws ClassNotFoundException     {       System.out.println("");       System.out.println("================== Find System Class " + name + " ====================");         check();       ClassLoader system = getSystemClassLoader();       if (system == null) {         if (!checkName(name, true)){           System.out.println("### Exception : Class Not Found!");           throw new ClassNotFoundException(name);         }         System.out.println("### find bootstrap class ...");         return findBootstrapClass(name);       }       System.out.println("### load class ...");       return system.loadClass(name);       }

 

protected final void resolveClass(Class c) {         System.out.println("==> ==> resolveClass() begin");       check();       resolveClass0(c);         System.out.println("==> ==> resolveClass() end");     }

 

protected synchronized Class loadClass(String name, boolean resolve)       throws ClassNotFoundException     {       // First, check if the class has already been loaded         System.out.println("");         System.out.println("================== Load Class " + name + " ====================");         Class c = findLoadedClass(name);       if (c == null) {           try {              if (parent != null) {                  c = parent.loadClass(name, false);              } else {                  c = findBootstrapClass0(name);              }           } catch (ClassNotFoundException e) {               System.out.println("### Catch Exception : Class Not Found Exception");               // If still not found, then invoke findClass in order               // to find the class.               c = findClass(name);           }       }       if (resolve) {           System.out.println("### Resolve Class");           resolveClass(c);       }           if(c == null)           System.out.println("### Load null class, Return NULL!!!");         else           System.out.println("### Loaded class, Return from LoadClass()");         return c;     }

      

重新编译后打包，替换原rt.jar文件，再次执行，结果如下：

结果分析：

CheckFileB类在装载时发生错误，抛出ClassFormatError异常，也即CheckFileB并没有加载成功，所以并未执行。

 

 

 

测试

1、查看机器jdk中%JAVA_HOME%/lib/rt.jar是否被修改过。