CSB541 Network Programming 網路程式設計. Ch.2 Streams 吳俊興國立高雄大學資訊工程學系

Transcription

1 CSB541 Network Programming 網路程式設計 Ch.2 Streams 吳俊興國立高雄大學資訊工程學系

2 Outline 2.1 Output Streams 2.2 Input Streams 2.3 Filter Streams 2.4 Readers and Writers 2

3 Java I/O Built on streams I/O in Java is organized differently than it is in most other languages Input streams read data; output streams write data Different stream classes provided, same basic methods Examples File stream: Java.io.FileInputStream Network streams: sun.net.telnetoutputstream After a stream is created, you can often ignore the details of exactly what it is you re reading or writing Filter streams can be chained To modify the data as it s read or written i.e. encrypting, compressing, converting, encoding Readers and writers can be chained to input and output streams to allow programs to read and write text (i.e., characters) rather than bytes Streams are Synchronous Wait for the data to be read or written before doing anything else I/O can be slow, so try to put I/O in its own thread for doing anything else Nonblocking I/O also offered using channels and buffers (Ch. 11) 3

4 2.1 Output Streams java.io.outputstream: the superclass of all classes representing an output stream of bytes int b: only eight bits (0-255) are placed! Subclass examples: FileOutputStream, TelnetOutputStream ByteArrayOutputStream: write data into an expandable byte array 4

5 Example: Character-Generator Server Given The printable ASCII characters are those between 33 and 126 numberofcharactersperline = 72 Output: multiple lines of printable ASCII characters like this Infinite loop 5

6 Sample Code 1: byte-by-byte The character-generator server will terminate only when the client closes the connection. The Java code will see this as an IOException Bytes are written onto out one at a time How many times are write() called to output a line? 6

7 Sample Code 2: line-by-line write(byte[] data): write a line of byte array 7

8 Buffered Stream in Java BufferedOutputStream and BufferedWriter support buffered output directly Remember to flush the output stream by the flush() method! 8

9 Initialize and Close Remember to close a stream to prevent resource leak Dispose pattern: for any object that needs to be cleaned up before it s garbage collected Java6+ Avoid NullPointerExceptions Java 7+: try with resources 9

10 2.2 Input Streams java.io.inputstream: the superclass of all classes representing an input stream of bytes A single byte (0-255) or EndOfStream (-1) Fill input array starting at offset up to length bytes Subclass examples: Skip over data without reading FileInputStream, TelnetInputStream ByteArrayInputStream: read data from an array of bytes cast to byte required for int return of read() Note: byte is signed (-128 to 127); Beware of casting back to int 10

11 Java Primitive Data Types Type Internal Range byte 8-bit signed 2 s complement -128 ~ 127 short 16-bit signed 2 s complement ~ int 32-bit signed 2 s complement ~ long 64-bit signed 2 s complement ~ float 32-bit IEEE 754 floating point ±1.4E-45~± E+38 double 64-bit IEEE 754 floating point ±4.9E-324~ ± E+308 char single 16-bit Unicode character '\u0000 ~'\uffff' (Unicode) boolean 8-bit, two values true, false Java doesn t support unsigned integers natively For simplicity Further: Number and String classes String: a sequence of characters 11

12 Number class in the java.lang package The Java platform provides wrapper classes for each of the primitive data types done by the compiler if you use a primitive where an object is expected, the compiler boxes the primitive in its wrapper class for you Similarly, if you use a number object when a primitive is expected, the compiler unboxes the object for you Three reasons to use a Number object rather than a primitive As an argument of a method that expects an object (often used when manipulating collections of numbers) To use constants defined by the class, such as MIN_VALUE and MAX_VALUE, that provide the upper and lower bounds of the data type. To use class methods for converting values to and from other primitive types for converting to and from strings, and for converting between number systems (decimal, octal, hexadecimal, binary) 12

13 Read Less Problem of (Network) Streams Reading N bytes from a network connection may return less bytes Data with no more than 1024 bytes are possible Read loop is required to guarantee the expected length of bytes Potential Issue: Hang (loop) if 1024 bytes might never arrive Read loop with testing Read exactly available 13

14 Additional Methods of InputStream Class: Marking and Resetting mark() marks the current position in this input stream, and subsequent call to reset() repositions this stream at the last marked position, so that subsequent read() can re-read the same up to readaheadlimit bytes marksupported() tests if the input stream supports mark() and reset() If you try to reset too far back, an IOException is thrown There can be only one mark in a stream at any given time Marking a second location erases the first mark The only input stream classes in java.io that always support marking are BufferedInputStream and ByteArrayInputStream Author s opinion: very poor design of Java More streams don t support these, so it s better to place these three methods in a separate interface rather in the InputStream class 14

15 2.3 Filter Streams InputStream and OutputStream are fairly raw/primitive classes Java provides a number of filter classes you can attach to raw streams to translate the raw bytes to and from these and other formats Two versions of filters The filter streams Still work primarily with raw data as bytes: by compressing the data or interpreting it as binary numbers i.e. System.out The readers and writers Handle the special case of text in a variety of encodings such as UTF-8 and ISO Filters are organized in a chain. Each link in the chain Receives data from the previous filter or stream, and Passes the data along to the next link in the chain 15

16 Example: Flow of Data through a Chain of Filters Finally, the text is read into the application and processed An InputStreamReader converts the decompressed data to Unicode text A GZIPInputStream decompresses the deciphered data A CipherInput Stream decrypts the data. A BufferedInputStream buffers the data to speed up the entire process A compressed, encrypted text file arrives from the local network interface, where native code presents it to the undocumented TelnetInputStream 16

17 Chaining Filters Together Filters are connected to streams by their constructors Example: buffers input from the file data.txt Methods of both fin and bin can be used, but you should only use the last filter in the chain to do the actual reading or writing However, sometimes you may need to use the methods of multiple filters in a chain Example: construct one stream directly inside another Connection is permanent Filters cannot be disconnected from a stream 17

18 Buffered Streams The BufferedOutputStream class Stores written data in a buffer (a protected byte array field named buf) until the buffer is full or the stream is flushed Then it writes the data onto the underlying output stream all at once A single write of many bytes is almost always much faster than many small writes that add up to the same thing Especially true of network connections because each TCP segment or UDP packet carries a finite amount of overhead, generally about 40 bytes worth The BufferedInputStream class When one of the stream s read() methods is called, it first tries to get the requested data from the buffer Only when the buffer runs out of data does the stream read from the underlying source It reads as much data as it can from the source into the buffer, whether it needs all the data immediately or not Buffering can substantially improve performance for reading files from a local disk, but the gain is less obvious on network connections where the bottleneck is often the speed at which the network can deliver data 18

19 Constructors of Buffered Streams Arguments of two constructors The underlying stream The number of bytes in the buffer Default values: 2048 bytes for an input stream and 512 bytes for an output stream For network connections, it s hard to predict and it varies You want something a little larger than the typical packet size» TCP/IP will chunk or fragment data into suitable size for underlying links Faster, higher-bandwidth networks tend to use larger packets Beware of using buffered streams read() methods of BufferedInputStream Return only when the array or subarray has been completely filled the end of stream is reached, or the underlying stream would block on further reads Most input streams read from the underlying stream only once It is essential to flush the BufferedOutputStream when you reach a point at which the data needs to be sent 19

20 PrintStream Adds the ability to Print representations of various data values to another output stream System.out is a PrintStream, a filter output stream 9 overloaded print() methods and 10 overloaded println() methods Println(): append a platform-dependent line separator» Linefeed (\n) on Unix, a carriage return (\r) on Mac OS 9, and (\r\n) on Windows Most network protocols such as HTTP and Gnutella also terminate lines with \r\n There are no problems with println() for Java programs on Windows, but fail on Unix and the Mac Assumes the default encoding of the platform on which it s running The expected encodings may be mismatched between the server and the clients Never throws an IOException The code must explicitly check flag using the checkerror() method after every call Not good for network programs that must be prepared to deal with unexpected interruptions in the flow of data Be created to flush automatically The flush method is automatically invoked after a byte array is written, one of the println methods is invoked, or a newline character or byte is written 20

21 Data Streams The DataInputStream and DataOutputStream classes Providing methods for reading and writing Java s primitive data types and strings in a binary format Intended for exchanging data between two different Java programs through a network connection, a data file, a pipe, or some other intermediary All data is written in big-endian format Two s-complement integers IEEE 754 floating-point numbers A single byte Boolean with the value 0 for false and 1 for true Unicode For exchanging UTF-8 text with all other software, you should use an InputStreamReader with the appropriate encoding 21

22 DataOutputStream Class Match DataInputStream writechars(): writing each character in turn as a two-type, big-endian Unicode character writebytes(): writes only the least significant byte of each character Information will be lost for any string with characters from outside the Latin-1 character set writeutf(): encodes the string itself in a variant of the UTF-8 encoding of Unicode Subtly incompatible with most non-java software Should be used only for exchanging data with other Java program For exchanging UTF-8 text with all other software, should use an InputStreamReader with appropriate encoding 22

23 DataInputStream Class 9 methods match DataOutputStream 2 methods to read unsigned bytes and unsigned shorts 2 methods to repeatedly read data until the requested bytes have been read. If not enough, an IOException is thrown readline() to read a line of text is provided, but deprecated Do not always recognize a single carriage return as ending a line Handled by the readline() method of the BufferedReader class instead 23

24 2.4 Readers and Writers Java provides an almost complete mirror of the I/O stream class hierarchy designed for working with characters instead of bytes Many programmers have a bad habit of writing code as if all text were ASCII or at least in the native encoding (i.e. Big-5) of the platform Java s native character set is the UTF-16 encoding of Unicode java.io.reader and java.io.writer: two abstract superclasses for reading and writing characters Like java.io.inputstream and java.io.outputstream Never used directly Two most important concrete subclass: InputStreamReader and OutputStreamWriter Translate between bytes and Unicode characters Several raw reader and writer classes FileReader/FileWriter: work with files StringReader/StringWriter, CharArrayReader/CharArrayWriter: work inside Java 24

25 Writers The Writer class mirrors the java.io.outputstream class A subclass must override the three base methods write(char [] text, int offset, int length), flush(), close() Various write() methods: given a Writer object w The exact output depends on the encoding w uses Big-endian UTF E F B Little-endian UTF-16 4E F B 00 Latin-1,UTF-8 4E F726B 25

26 OutputStreamWriter The most important concrete subclass of Writer Receives characters from a Java program Converts these into bytes according to a specified encoding Writes them onto an underlying output stream Constructor: specifies the output stream and the encoding If no encoding is specified, the default encoding for the platform is used It s better to explicitly specify the character set It has only the usual Writer methods, and one getencoding() method to return the encoding of the object 26

27 Readers The Reader class mirrors the java.io.inputstream class A subclass must override the two base methods read(char [] text, int offset, int length), close() The read() method return a single Unicode character as an int with a value from 0 to 65,535 or -1 on end of stream New ready() method: returns a boolean indicating whether the reader may be read without blocking available() returns an int specifying a minimum number of bytes that may be read without blocking Since it s hard to determine the length for some character encodings 27

28 InputStreamReader The most important concrete subclass of Reader Reads bytes from an underlying input stream such as FileInputStream Converts these into characters according to a specified encoding Return them Constructor: specifies the input stream and the encoding If no encoding is specified, the default encoding for the platform is used If an unknown encoding is specified, an UnsupportedEncodingException is thrown Example: getmaccyrillicstring() converts MacCyrillic to Unicode 28

29 Filter Readers and Writers java.io.filterreader and java.io.filterwriter classes Additional character-oriented filters can be layered on InputStreamReader and OutputStreamWriter classes, which change the interface from a byteoriented interface to a character-oriented interface Subclasses perform specific filtering BufferedReader/BufferedWriter: use an internal array of chars as a buffer Similar to BufferedInputStream/BufferedOutputStream, but supporting encoding conversion and platform-dependent line-separator Constructors: If the size is not set, the default size of 8,192 characters is used readline() of BufferedReader: reads a single line of text and return it as astring newline() of BufferedWriter: inserts a platform-dependent line-separator string LineNumberReader: a buffered character-input stream that keeps track of line numbers PushbackReader: a character-stream reader that allows characters to be pushed back into the stream PrintWriter: replacement for PrintStream 29

30 getmaccyrillicstring() with BufferedReader getmaccyrillicstring() converts MacCyrillic to Unicode Un-buffered Buffered: run faster 30

31 PrintWriter A replacement for Java 1.0 s PrintStream class that properly handles multibyte character sets and international text Has an almost identical collection of methods to PrintStream New code should use PrintWriter instead of PrintStream 31

32 Summary Byte-oriented I/O (ASCII encoding assumed) java.io.outputstream (Abstract superclass): write sequence of bytes BufferedOutputStream: writing data in a buffer PrintStream: System.out FileOutputStream DataOutputStream: writing primitive data types and strings in a binary format java.io.inputstream (Abstract superclass) BufferedInputStream, FileInputStream, DataInputStream Character-oriented I/O (UTF-16 default encoding) java.io.writer (Abstract superclass) OutputStreamWriter FileWriter, StringWriter, CharArrayWriter BufferedWriter PrintWriter (replacement for PrintStream) java.io.reader (Abstract superclass) InputStreamReader FileReader, StringReader, CharArrayReader BufferedReader LineNumberReader: a buffered character-input stream that keeps track of line numbers PushbackReader: a character-stream reader that allows characters to be pushed back into the stream 32