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);
    }
}     转自：http://jackycheng2007.iteye.com/blog/1473625