Java System 种详解 - in, out, err

Java System 类详解 - in, out, err
几乎所有的都用过这个System类吧，因为大家学习的第一个语句大概就是

package jdk.lang;public class SystemInspection {public static void main(String[] args) throws Exception {System.out.println(“Hello World”);}}

本文将一一详解这个类的标准输入(in)，输出(out)和错误流(err)使用和原理。先看一个更复杂的例子：

package jdk.lang;public class SystemIOE {public static void main(String[] args) throws Exception {System.out.println("Hello Out");System.err.println("Hello Error");byte[] b = new byte[1024];int count = System.in.read(b);System.out.println(new String(b, 0, count));}}

在eclipse上运行，输出：

Hello Out
Hello Error
Hello In 
Hello In

分析：

其实out, in 和 err是它的静态属性：

public final static InputStream in = nullInputStream();public final static PrintStream out = nullPrintStream();public final static PrintStream err = nullPrintStream();

他们其实是在private static void initializeSystemClass()中初始化的：

FileInputStream fdIn = new FileInputStream(FileDescriptor.in);FileOutputStream fdOut = new FileOutputStream(FileDescriptor.out);FileOutputStream fdErr = new FileOutputStream(FileDescriptor.err);setIn0(new BufferedInputStream(fdIn));setOut0(new PrintStream(new BufferedOutputStream(fdOut, 128), true));setErr0(new PrintStream(new BufferedOutputStream(fdErr, 128), true));

FileDescriptor.out，FileDescriptor.in和FileDescriptor.err其实是

public static final FileDescriptor in = standardStream(0);public static final FileDescriptor out = standardStream(1);public static final FileDescriptor err = standardStream(2);

再往下就是native的调用了。
同时System类还提供方法让你设置in，out和err，这样你就可以实现自己的日志系统了。运行下面的代码：

package jdk.lang;import java.io.BufferedInputStream;import java.io.File;import java.io.FileInputStream;import java.io.FileNotFoundException;import java.io.FileOutputStream;import java.io.IOException;import java.io.InputStream;import java.io.PrintStream;public class SystemIOE {public static void main(String[] args) throws Exception {redirect();helloIO();}private static void helloIO() throws IOException {System.out.println("Hello Out");System.err.println("Hello Error");byte[] b = new byte[1024];int count = System.in.read(b);System.out.println(new String(b, 0, count));}public static void redirect() throws FileNotFoundException {InputStream in = new BufferedInputStream(new FileInputStream(new File("c:/in.txt")));System.setIn(in);PrintStream out = new PrintStream(new FileOutputStream(new File("c:/out.log")));System.setOut(out);PrintStream err = new PrintStream(new FileOutputStream(new File("c:/err.log")));System.setErr(err);}}

后面我再看看logback的实现是不是用到了这个类。待续。。。

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