Working with Streams 使用流

Working with Streams

Along with File and RandomAccessFile, Java uses streams to perform I/O operations. A stream is an

ordered sequence of bytes of arbitrary length. Bytes flow over an output stream from an application

to a destination and flow over an input stream from a source to an application. Figure 11-2 illustrates

these flows.
application write  destination

                  * * *

         output stream

source  read  application          

             * * *

        input stream

Figure 11-2. Conceptualizing output and input streams as flows of bytes
Note Java’s use of the word stream is analogous to stream of water, stream of electrons, and so on.

注意 Java的单词stream 的使用类似于水流、电流等等。

Java recognizes various stream destinations, such as byte arrays, files, screens, sockets (network

endpoints), and thread pipes. Java also recognizes various stream sources. Examples include byte

arrays, files, keyboards, sockets, and thread pipes. (I will discuss sockets in Chapter 12.)

Stream Classes Overview

Stream Classes纵览

The java.io package provides several output stream and input stream classes that are descendants

of the abstract OutputStream and InputStream classes. Figure 11-3 reveals the hierarchy of output

stream classes.

Figure 11-3. All output stream classes except for PrintStream are denoted by their OutputStream suffixes

Figure 11-4 reveals the hierarchy of input stream classes.

Figure 11-4. Note that LineNumberInputStream and StringBufferInputStream are deprecated

LineNumberInputStream and StringBufferInputStream have been deprecated because they don’t

support different character encodings, a topic I discuss later in this chapter. LineNumberReader and

StringReader are their replacements. (I discuss readers later in this chapter.)

Note PrintStream is another class that should be deprecated because it doesn’t support different

character encodings; PrintWriter is its replacement. However, it’s doubtful that Oracle (and Google) will

deprecate this class because PrintStream is the type of the System class’s out and err class fields, and

too much legacy code depends upon this fact.

然而，不能肯定Oracle(或 Google)会弃用这个类，因为PrintStream是System类的out和err 类字段，并且太多遗产代码依赖这个事实。

Other Java packages provide additional output stream and input stream classes. For example,

java.util.zip provides four output stream classes that compress uncompressed data into various

formats and four matching input stream classes that uncompress compressed data from the same

formats:

 CheckedOutputStream

 CheckedInputStream

 DeflaterOutputStream

 GZIPOutputStream

 GZIPInputStream

 InflaterInputStream

 ZipOutputStream

 ZipInputStream

Also, the java.util.jar package provides a pair of stream classes for writing content to a JAR file

and for reading content from a JAR file:

 JarOutputStream

 JarInputStream

In the next several sections, I take you on a tour of most of java.io’s output stream and input stream

classes, beginning with OutputStream and InputStream.

OutputStream and InputStream

Java provides the OutputStream and InputStream classes for performing stream I/O. OutputStream is

the superclass of all output stream subclasses. Table 11-7 describes OutputStream’s methods.

Table 11-7. OutputStream Methods

Method Description

void close() Closes this output stream and releases any platform resources associated with the

stream. This method throws IOException when an I/O error occurs.

void flush() Flushes this output stream by writing any buffered output bytes to the destination.

If the intended destination of this output stream is an abstraction provided by the

underlying platform (for example, a file), flushing the stream only guarantees that

bytes previously written to the stream are passed to the underlying platform for

writing; it doesn’t guarantee that they’re actually written to a physical device such

as a disk drive. This method throws IOException when an I/O error occurs.

void write(byte[] b) Writes b.length bytes from byte array b to this output stream. In general, write(b)

behaves as if you specified write(b, 0, b.length). This method throws

NullPointerException when b is null and IOException when an I/O error occurs.

void write(byte[] b,int off, int len)

Writes len bytes from byte array b starting at offset off to this output stream.

This method throws NullPointerException when b is null; java.lang.

IndexOutOfBoundsException when off is negative, len is negative, or off + len is

greater than b.length; and IOException when an I/O error occurs.

void write(int b) Writes byte b to this output stream. Only the 8 low-order bits are written; the 24

high-order bits are ignored. This method throws IOException when an I/O error

occurs.

Note The close() method automatically flushes the output stream. If an application ends before close()

is called, the output stream is automatically closed and its data is flushed.

InputStream is the superclass of all input stream subclasses. Table 11-8 describes InputStream’s

methods.

Table 11-8. InputStream Methods

Method Description

int available() Returns an estimate of the number of bytes that can be read from this input stream

via the next read() method call (or skipped over via skip()) without blocking the

calling thread. This method throws IOException when an I/O error occurs.

It’s never correct to use this method’s return value to allocate a buffer for holding all

of the stream’s data because a subclass might not return the total size of the stream.

void close() Closes this input stream and releases any platform resources associated with the

stream. This method throws IOException when an I/O error occurs.

void mark(int

readlimit)

Marks the current position in this input stream. A subsequent call to reset()

repositions this stream to the last marked position so that subsequent read

operations re-read the same bytes. The readlimit argument tells this input stream

to allow that many bytes to be read before invalidating this mark (so that the stream

cannot be reset to the marked position).

boolean

markSupported()

Returns true when this input stream supports mark() and reset(); otherwise,

returns false.

int read() Reads and returns (as an int in the range 0 to 255) the next byte from this input

stream, or returns -1 when the end of the stream is reached.
This method blocks until

input is available, the end of the stream is detected, or an exception is thrown.

It throws IOException when an I/O error occurs.

在输入数据可用、检测到流末尾或者抛出异常前，此方法一直阻塞。

int read(byte[] b) Reads some number of bytes from this input stream and stores them in byte array b.

Returns the number of bytes actually read (which might be less than b’s length but is

never more than this length), or returns -1 when the end of the stream is reached (no

byte is available to read). This method blocks until input is available, the end of the

stream is detected, or an exception is thrown. It throws NullPointerException when

b is null and IOException when an I/O error occurs.

int read(byte[] b,int off, int len)

Reads no more than len bytes from this input stream and stores them in byte array

b, starting at the offset specified by off. Returns the number of bytes actually read

(which might be less than len but is never more than len), or returns -1 when the

end of the stream is reached (no byte is available to read). This method blocks until

input is available, the end of the stream is detected, or an exception is thrown. It

throws NullPointerException when b is null; IndexOutOfBoundsException when off

is negative, len is negative, or len is greater than b.length - off; and IOException

when an I/O error occurs.

void reset() Repositions this input stream to the position at the time mark() was last called. This

method throws IOException when this input stream has not been marked or the mark

has been invalidated.

long skip(long n) Skips over and discards n bytes of data from this input stream. This method might

skip over some smaller number of bytes (possibly zero), for example, when the end

of the file is reached before n bytes have been skipped. The actual number of bytes

skip
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ped is returned. When n is negative, no bytes are skipped. This method throws

IOException when this input stream doesn’t support skipping or when some other
I/O error occurs.