9-I/O · guava · 看云

# 9-I/O # 9-I/O [原文链接](http://code.google.com/p/guava-libraries/wiki/IOExplained) [译文链接](http://ifeve.com/google-guava-io) **译者：**沈义扬 ## 字节流和字符流 Guava使用术语”流” 来表示可关闭的，并且在底层资源中有位置状态的I/O数据流。术语”字节流”指的是InputStream或OutputStream，”字符流”指的是Reader 或Writer（虽然他们的接口Readable 和Appendable被更多地用于方法参数）。相应的工具方法分别在[`ByteStreams`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/io/ByteStreams.html) 和[`CharStreams`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/io/CharStreams.html)中。大多数Guava流工具一次处理一个完整的流，并且/或者为了效率自己处理缓冲。还要注意到，接受流为参数的Guava方法不会关闭这个流：关闭流的职责通常属于打开流的代码块。其中的一些工具方法列举如下： **ByteStreams****CharStreams**[`byte[] toByteArray(InputStream)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/io/ByteStreams.html#toByteArray%28java.io.InputStream%29)[`String toString(Readable)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/io/CharStreams.html#toString%28java.lang.Readable%29)N/A[`List<String> readLines(Readable)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/CharStreams.html#readLines%28java.lang.Readable%29)[`long copy(InputStream, OutputStream)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/io/ByteStreams.html#copy%28java.io.InputStream,%20java.io.OutputStream%29)[`long copy(Readable, Appendable)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/io/CharStreams.html#copy%28java.lang.Readable,%20java.lang.Appendable%29)[`void readFully(InputStream, byte[])`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/io/ByteStreams.html#readFully%28java.io.InputStream,%20byte%5B%5D%29)N/A[`void skipFully(InputStream, long)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/ByteStreams.html#skipFully%28java.io.InputStream,%20long%29)[`void skipFully(Reader, long)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/CharStreams.html#skipFully%28java.io.Reader,%20long%29)[`OutputStream nullOutputStream()`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/ByteStreams.html#nullOutputStream%28%29)[`Writer nullWriter()`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/CharStreams.html#nullWriter%28%29)**关于InputSupplier 和OutputSupplier要注意：** 在ByteStreams、CharStreams以及com.google.common.io包中的一些其他类中，某些方法仍然在使用InputSupplier和OutputSupplier接口。这两个借口和相关的方法是不推荐使用的：它们已经被下面描述的source和sink类型取代了，并且最终会被移除。 ## 源与汇通常我们都会创建I/O工具方法，这样可以避免在做基础运算时总是直接和流打交道。例如，Guava有Files.toByteArray(File) 和Files.write(File, byte\[\])。然而，流工具方法的创建经常最终导致散落各处的相似方法，每个方法读取不同类型的源或写入不同类型的汇\[sink\]。例如，Guava中的Resources.toByteArray(URL)和Files.toByteArray(File)做了同样的事情，只不过数据源一个是URL，一个是文件。为了解决这个问题，Guava有一系列关于源与汇的抽象。源或汇指某个你知道如何从中打开流的资源，比如File或URL。源是可读的，汇是可写的。此外，源与汇按照字节和字符划分类型。 **字节****字符****读**[`ByteSource`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/io/ByteSource.html)[`CharSource`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/io/CharSource.html)**写**[`ByteSink`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/io/ByteSink.html)[`CharSink`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/io/CharSink.html)源与汇API的好处是它们提供了通用的一组操作。比如，一旦你把数据源包装成了ByteSource，无论它原先的类型是什么，你都得到了一组按字节操作的方法。 ### 创建源与汇 Guava提供了若干源与汇的实现： **字节****字符**[`Files.asByteSource(File)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/Files.html#asByteSource%28java.io.File%29)[`Files.asCharSource(File, Charset)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/Files.html#asCharSource%28java.io.File,%20java.nio.charset.Charset%29)[`Files.asByteSink(File, FileWriteMode...)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/Files.html#asByteSink%28java.io.File,%20com.google.common.io.FileWriteMode...%29)[`Files.asCharSink(File, Charset, FileWriteMode...)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/Files.html#asCharSink%28java.io.File,%20java.nio.charset.Charset,%20com.google.common.io.FileWriteMode...%29)[`Resources.asByteSource(URL)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/Resources.html#asByteSource%28java.net.URL%29)[`Resources.asCharSource(URL, Charset)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/Resources.html#asCharSource%28java.net.URL,%20java.nio.charset.Charset%29)[`ByteSource.wrap(byte[])`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/ByteSource.html#wrap%28byte%5B%5D%29)[`CharSource.wrap(CharSequence)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/CharSource.html#wrap%28java.lang.CharSequence%29)[`ByteSource.concat(ByteSource...)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/ByteSource.html#concat%28com.google.common.io.ByteSource...%29)[`CharSource.concat(CharSource...)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/CharSource.html#concat%28com.google.common.io.CharSource...%29)[`ByteSource.slice(long, long)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/ByteSource.html#slice%28long,%20long%29)N/AN/A[`ByteSource.asCharSource(Charset)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/ByteSource.html#asCharSource%28java.nio.charset.Charset%29)N/A[`ByteSink.asCharSink(Charset)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/ByteSink.html#asCharSink%28java.nio.charset.Charset%29)此外，你也可以继承这些类，以创建新的实现。注：把已经打开的流（比如InputStream）包装为源或汇听起来是很有诱惑力的，但是应该避免这样做。源与汇的实现应该在每次openStream()方法被调用时都创建一个新的流。始终创建新的流可以让源或汇管理流的整个生命周期，并且让多次调用openStream()返回的流都是可用的。此外，如果你在创建源或汇之前创建了流，你不得不在异常的时候自己保证关闭流，这压根就违背了发挥源与汇API优点的初衷。 ### 使用源与汇一旦有了源与汇的实例，就可以进行若干读写操作。 **通用操作** 所有源与汇都有一些方法用于打开新的流用于读或写。默认情况下，其他源与汇操作都是先用这些方法打开流，然后做一些读或写，最后保证流被正确地关闭了。这些方法列举如下： - openStream()：根据源与汇的类型，返回InputStream、OutputStream、Reader或者Writer。 - openBufferedStream()：根据源与汇的类型，返回InputStream、OutputStream、BufferedReader或者BufferedWriter。返回的流保证在必要情况下做了缓冲。例如，从字节数组读数据的源就没有必要再在内存中作缓冲，这就是为什么该方法针对字节源不返回BufferedInputStream。字符源属于例外情况，它一定返回BufferedReader，因为BufferedReader中才有readLine()方法。 **源操作** **字节源****字符源**[`byte[] read()`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/ByteSource.html#read%28%29)[`String read()`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/CharSource.html#read%28%29)N/A[`ImmutableList<String> readLines()`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/CharSource.html#readLines%28%29)N/A[`String readFirstLine()`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/CharSource.html#readFirstLine%28%29)[`long copyTo(ByteSink)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/ByteSource.html#copyTo%28com.google.common.io.ByteSink%29)[`long copyTo(CharSink)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/CharSource.html#copyTo%28com.google.common.io.CharSink%29)[`long copyTo(OutputStream)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/ByteSource.html#copyTo%28java.io.OutputStream%29)[`long copyTo(Appendable)` ](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/CharSource.html#copyTo%28java.lang.Appendable%29)[`long size()`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/ByteSource.html#size%28%29) (in bytes)N/A[`boolean isEmpty()`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/ByteSource.html#isEmpty%28%29)[`boolean isEmpty()`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/CharSource.html#isEmpty%28%29)[`boolean contentEquals(ByteSource)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/ByteSource.html#contentEquals%28com.google.common.io.ByteSource%29)N/A[`HashCode hash(HashFunction)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/ByteSource.html#hash%28com.google.common.hash.HashFunction%29)N/A汇操作 **字节汇****字符汇**[`void write(byte[])`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/ByteSink.html#write%28byte%5B%5D%29)[`void write(CharSequence)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/CharSink.html#write%28java.lang.CharSequence%29)[`long writeFrom(InputStream)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/ByteSink.html#writeFrom%28java.io.InputStream%29)[`long writeFrom(Readable)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/CharSink.html#writeFrom%28java.lang.Readable%29)N/A[`void writeLines(Iterable<? extends CharSequence>)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/CharSink.html#writeLines%28java.lang.Iterable%29)N/A[`void writeLines(Iterable<? extends CharSequence>, String)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/CharSink.html#writeLines%28java.lang.Iterable,%20java.lang.String%29)### 范例 ``` <pre class="calibre11">``` //Read the lines of a UTF-8 text file ImmutableList<String> lines = Files.asCharSource(file, Charsets.UTF_8).readLines(); //Count distinct word occurrences in a file Multiset<String> wordOccurrences = HashMultiset.create( Splitter.on(CharMatcher.WHITESPACE) .trimResults() .omitEmptyStrings() .split(Files.asCharSource(file, Charsets.UTF_8).read())); //SHA-1 a file HashCode hash = Files.asByteSource(file).hash(Hashing.sha1()); //Copy the data from a URL to a file Resources.asByteSource(url).copyTo(Files.asByteSink(file)); ``` ``` ## 文件操作除了创建文件源和文件的方法，Files类还包含了若干你可能感兴趣的便利方法。 [`createParentDirs(File)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/io/Files.html#createParentDirs%28java.io.File%29)必要时为文件创建父目录[`getFileExtension(String)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/io/Files.html#getFileExtension%28java.lang.String%29)返回给定路径所表示文件的扩展名[`getNameWithoutExtension(String)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/Files.html#getNameWithoutExtension%28java.lang.String%29)返回去除了扩展名的文件名[`simplifyPath(String)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/io/Files.html#simplifyPath%28java.lang.String%29)规范文件路径，并不总是与文件系统一致，请仔细测试[`fileTreeTraverser()`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/Files.html#fileTreeTraverser%28%29)返回TreeTraverser用于遍历文件树