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。。。

评论

1 楼 xunke515 2014-08-30  

有启发.感谢