The package contains a class CRC32 that computes the checksum of a sequence of bytes, using the following loop: var crc = new CRC32() Corruption of a file makes it very likely that the checksum has changed. That checksum is often used to determine whether a file has been corrupted. Listing 2.5 computes the 32-bit cyclic redundancy checksum (CRC32) of a file.
To write numbers to a buffer, use one of the methods putInt putCharĪt some point, and certainly when the channel is closed, these changes are written back to the file. This pair of methods does not use the set/ get naming convention. To find out the current byte order of a buffer, call ByteOrder b = buffer.order() However, if you need to process a file containing binary numbers in little-endian order, simply call buffer.order(ByteOrder.LITTLE_ENDIAN) As we already mentioned, Java uses big-endian ordering for binary data. To read primitive-type values that are stored as binary values in the file. You can also read and write arrays of bytes with the methods get(byte bytes)įinally, there are methods getInt getChar For example, you can sequentially traverse all bytes in the buffer as while (buffer.hasRemaining())Īlternatively, you can use random access: for (int i = 0 i < buffer.limit() i++) A buffer has a position that is advanced by get and put operations. Once you have the buffer, you can read and write data using the methods of the ByteBuffer class and the Buffer superclass.īuffers support both sequential and random data access. The exact behavior of simultaneous file mapping by multiple programs depends on the operating system.į: The resulting buffer is writable, but any changes are private to this buffer and not propagated to the file. Note that other programs that have mapped the same file might not see those changes immediately. Any attempt to write to the buffer results in a ReadOnlyBufferException.į_WRITE: The resulting buffer is writable, and the changes will be written back to the file at some time. Three modes are supported:į_ONLY: The resulting buffer is read-only. Specify the area of the file that you want to map and a mapping mode. Then, get a ByteBuffer from the channel by calling the map method of the FileChannel class. FileChannel channel = FileChannel.open(path, options) A channel is an abstraction for a disk file that lets you access operating system features such as memory mapping, file locking, and fast data transfers between files.
Here is what you do.įirst, get a channel for the file. The java.nio package makes memory mapping quite simple. For sequential reading of files of moderate size, on the other hand, there is no reason to use memory mapping. Of course, the exact values will differ greatly from one machine to another, but it is obvious that the performance gain, compared to random access, can be substantial. Table 2.5 Timing Data for File OperationsĪs you can see, on this particular machine, memory mapping is a bit faster than using buffered sequential input and dramatically faster than using a RandomAccessFile.
On one machine, we got the timing data shown in Table 2.5 when computing the checksum of the 37MB file rt.jar in the jre/lib directory of the JDK. 2.5.1 Memory-Mapped File PerformanceĪt the end of this section, you can find a program that computes the CRC32 checksum of a file using traditional file input and a memory-mapped file. Then the file can be accessed as if it were an in-memory array, which is much faster than the traditional file operations. Most operating systems can take advantage of a virtual memory implementation to “map” a file, or a region of a file, into memory. Core Java, Volume II-Advanced Features, 11th Edition
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