The Caesar. So that we do not confuse the regular class loader, we use a different extension,. To decrypt, the class loader simply subtracts the key from every byte. In the companion code for this book, you will find four class files, encrypted with a key value of 3—the traditional choice.
To run the encrypted program, you need the custom class loader defined in our ClassLoaderTest program. Encrypting class files has a number of practical uses provided, of course, that you use a cipher stronger than the Caesar cipher.
Without the decryption key, the class files are useless. They can neither be executed by a standard virtual machine nor readily disassembled. This means that you can use a custom class loader to authenticate the user of the class or to ensure that a program has been paid for before it will be allowed to run. Of course, encryption is only one application of a custom class loader.
You can use other types of class loaders to solve other problems, for example, storing class files in a database. Class 1. ClassLoader 1.
In the name of the class, use. URLClassLoader 1. Thread 1. If no context class loader is set explicitly when a thread is started, the parent's context class loader is used. I would like to receive exclusive offers and hear about products from InformIT and its family of brands.
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Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site. Returns: true if and only if the file or directory denoted by this abstract pathname exists; false otherwise Throws: SecurityException - If a security manager exists and its SecurityManager. String method denies read access to the file or directory isDirectory public boolean isDirectory Tests whether the file denoted by this abstract pathname is a directory.
Returns: true if and only if the file denoted by this abstract pathname exists and is a directory; false otherwise Throws: SecurityException - If a security manager exists and its SecurityManager. String method denies read access to the file isFile public boolean isFile Tests whether the file denoted by this abstract pathname is a normal file. A file is normal if it is not a directory and, in addition, satisfies other system-dependent criteria.
Any non-directory file created by a Java application is guaranteed to be a normal file. Returns: true if and only if the file denoted by this abstract pathname exists and is a normal file; false otherwise Throws: SecurityException - If a security manager exists and its SecurityManager. String method denies read access to the file isHidden public boolean isHidden Tests whether the file named by this abstract pathname is a hidden file.
The exact definition of hidden is system-dependent. On UNIX systems, a file is considered to be hidden if its name begins with a period character '. On Microsoft Windows systems, a file is considered to be hidden if it has been marked as such in the filesystem. Returns: true if and only if the file denoted by this abstract pathname is hidden according to the conventions of the underlying platform Throws: SecurityException - If a security manager exists and its SecurityManager.
String method denies read access to the file Since: 1. String method denies read access to the file length public long length Returns the length of the file denoted by this abstract pathname.
The return value is unspecified if this pathname denotes a directory. Returns: The length, in bytes, of the file denoted by this abstract pathname, or 0L if the file does not exist. Some operating systems may return 0L for pathnames denoting system-dependent entities such as devices or pipes.
String method denies read access to the file createNewFile public boolean createNewFile throws IOException Atomically creates a new, empty file named by this abstract pathname if and only if a file with this name does not yet exist. The check for the existence of the file and the creation of the file if it does not exist are a single operation that is atomic with respect to all other filesystem activities that might affect the file.
Note: this method should not be used for file-locking, as the resulting protocol cannot be made to work reliably. The FileLock facility should be used instead.
String method denies write access to the file Since: 1. If this pathname denotes a directory, then the directory must be empty in order to be deleted. Note that the Files class defines the delete method to throw an IOException when a file cannot be deleted.
This is useful for error reporting and to diagnose why a file cannot be deleted. Returns: true if and only if the file or directory is successfully deleted; false otherwise Throws: SecurityException - If a security manager exists and its SecurityManager.
String method denies delete access to the file deleteOnExit public void deleteOnExit Requests that the file or directory denoted by this abstract pathname be deleted when the virtual machine terminates.
Files or directories are deleted in the reverse order that they are registered. Invoking this method to delete a file or directory that is already registered for deletion has no effect. Deletion will be attempted only for normal termination of the virtual machine, as defined by the Java Language Specification.
Once deletion has been requested, it is not possible to cancel the request. This method should therefore be used with care. String method denies delete access to the file Since: 1. If this abstract pathname does not denote a directory, then this method returns null.
Otherwise an array of strings is returned, one for each file or directory in the directory. Names denoting the directory itself and the directory's parent directory are not included in the result. Each string is a file name rather than a complete path. There is no guarantee that the name strings in the resulting array will appear in any specific order; they are not, in particular, guaranteed to appear in alphabetical order.
Note that the Files class defines the newDirectoryStream method to open a directory and iterate over the names of the files in the directory.
This may use less resources when working with very large directories, and may be more responsive when working with remote directories. Returns: An array of strings naming the files and directories in the directory denoted by this abstract pathname. The array will be empty if the directory is empty. The behavior of this method is the same as that of the list method, except that the strings in the returned array must satisfy the filter. If the given filter is null then all names are accepted.
Otherwise, a name satisfies the filter if and only if the value true results when the FilenameFilter. Parameters: filter - A filename filter Returns: An array of strings naming the files and directories in the directory denoted by this abstract pathname that were accepted by the given filter.
The array will be empty if the directory is empty or if no names were accepted by the filter. Otherwise an array of File objects is returned, one for each file or directory in the directory. Pathnames denoting the directory itself and the directory's parent directory are not included in the result.
Each resulting abstract pathname is constructed from this abstract pathname using the File File, String constructor. Therefore if this pathname is absolute then each resulting pathname is absolute; if this pathname is relative then each resulting pathname will be relative to the same directory. This may use less resources when working with very large directories. Returns: An array of abstract pathnames denoting the files and directories in the directory denoted by this abstract pathname.
The behavior of this method is the same as that of the listFiles method, except that the pathnames in the returned array must satisfy the filter.
If the given filter is null then all pathnames are accepted. Otherwise, a pathname satisfies the filter if and only if the value true results when the FilenameFilter. Parameters: filter - A filename filter Returns: An array of abstract pathnames denoting the files and directories in the directory denoted by this abstract pathname. Otherwise, a pathname satisfies the filter if and only if the value true results when the FileFilter. Parameters: filter - A file filter Returns: An array of abstract pathnames denoting the files and directories in the directory denoted by this abstract pathname.
Filter mkdir public boolean mkdir Creates the directory named by this abstract pathname. Returns: true if and only if the directory was created; false otherwise Throws: SecurityException - If a security manager exists and its SecurityManager.
String method does not permit the named directory to be created mkdirs public boolean mkdirs Creates the directory named by this abstract pathname, including any necessary but nonexistent parent directories. Note that if this operation fails it may have succeeded in creating some of the necessary parent directories.
Returns: true if and only if the directory was created, along with all necessary parent directories; false otherwise Throws: SecurityException - If a security manager exists and its SecurityManager. String method does not permit verification of the existence of the named directory and all necessary parent directories; or if the SecurityManager.
String method does not permit the named directory and all necessary parent directories to be created renameTo public boolean renameTo File dest Renames the file denoted by this abstract pathname. Many aspects of the behavior of this method are inherently platform-dependent: The rename operation might not be able to move a file from one filesystem to another, it might not be atomic, and it might not succeed if a file with the destination abstract pathname already exists.
The return value should always be checked to make sure that the rename operation was successful. Note that the Files class defines the move method to move or rename a file in a platform independent manner. Parameters: dest - The new abstract pathname for the named file Returns: true if and only if the renaming succeeded; false otherwise Throws: SecurityException - If a security manager exists and its SecurityManager.
String method denies write access to either the old or new pathnames NullPointerException - If parameter dest is null setLastModified public boolean setLastModified long time Sets the last-modified time of the file or directory named by this abstract pathname.
All platforms support file-modification times to the nearest second, but some provide more precision. The argument will be truncated to fit the supported precision.
If the operation succeeds and no intervening operations on the file take place, then the next invocation of the lastModified method will return the possibly truncated time argument that was passed to this method. Parameters: time - The new last-modified time, measured in milliseconds since the epoch GMT, January 1, Returns: true if and only if the operation succeeded; false otherwise Throws: IllegalArgumentException - If the argument is negative SecurityException - If a security manager exists and its SecurityManager.
String method denies write access to the named file Since: 1. After invoking this method the file or directory is guaranteed not to change until it is either deleted or marked to allow write access. Whether or not a read-only file or directory may be deleted depends upon the underlying system. Returns: true if and only if the operation succeeded; false otherwise Throws: SecurityException - If a security manager exists and its SecurityManager.
The Files class defines methods that operate on file attributes including file permissions. This may be used when finer manipulation of file permissions is required. Parameters: writable - If true , sets the access permission to allow write operations; if false to disallow write operations ownerOnly - If true , the write permission applies only to the owner's write permission; otherwise, it applies to everybody.
If the underlying file system can not distinguish the owner's write permission from that of others, then the permission will apply to everybody, regardless of this value. Returns: true if and only if the operation succeeded. JDK - Old bytecode verifier accepts illegal class. However, the HotSpot VM class verifier does not detect the problem, and loads the class.
The class is invalid because the StackMap for bytecode index 45 is incorrect. Bytecode index 45 is, in turn, protected by a handler at index However, we can disable these checks using java -noverify VerifyGeekFile One might think how a program could have been manipulated since compiler checks these above validations before generating.
But the class file is vulnerable to change before JVM loads it. The bytecode used in the class file is well documented and someone having some knowledge of hex can change the values of hex codes in the. For example : The Applets in the web browsers do not download source code instead they download precompiled class files.
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