听着张孝祥老师关于缓冲区知识的课,发现还是有一些没有掌握,动手试了一下,果然发现了问题。
先讲一下关于java缓冲区的知识,应用程序和IO设备之间存在一个缓冲区,一般流是没有缓冲区的,但是如果存在缓冲区,就会发现很大的问题。
错误代码如下:为了确保问题发生,我使用了BufferedOutputStream,使得手动构造出了一个缓冲区。
import java.io.*;public class Test { public static void main(String[] args) throws Exception{ DataOutputStream out = new DataOutputStream( new BufferedOutputStream( new FileOutputStream("1.txt"))); out.writeChars("hello"); FileInputStream in = new FileInputStream("1.txt"); int len = in.available(); byte[] b = new byte[len]; int actlen = in.read(b); String str = new String(b); System.out.println(str); } }
发现什么问题了吗?
因为如果没有缓冲区,应用程序每次IO都要和设备进行通信,效率很低,因此缓冲区为了提高效率,当写入设备时,先写入缓冲区,等到缓冲区有足够多的数据时,就整体写入设备。这就是问题所在,上个例子中,当我们写入hello时,由于hello占用空间很小,所以暂时存放在缓冲区中,后来输入流想要从文件中读取,但是由于文件中没有字节,所以不能读取hello。
这里,解决方法很简单,只要调用out.flush() 或者out.close()即可,这是把缓冲区的数据手动写入文件。
正确代码如下:
import java.io.*;public class Test { public static void main(String[] args) throws Exception{ DataOutputStream out = new DataOutputStream( new BufferedOutputStream( new FileOutputStream("1.txt"))); out.writeChars("hello"); out.close();//inserted FileInputStream in = new FileInputStream("1.txt"); int len = in.available(); byte[] b = new byte[len]; int actlen = in.read(b); String str = new String(b); System.out.println(str); } }
接下来又是我遇到的一个例子,这个例子也很明显的反应出缓冲区的问题。
扫描二维码关注公众号,回复:
6053803 查看本文章
import java.io.BufferedReader;import java.io.FileReader;import java.io.FileWriter;import java.io.IOException;import java.io.PrintWriter;import java.util.Calendar;import java.util.Date;import java.util.GregorianCalendar;import java.util.Scanner;import java.util.StringTokenizer;public class StringTokenizerTest { public static void main(String[] args) { Employee[] e = new Employee[3]; e[0] = new Employee("Carl Cracker", 75000, 1987, 12, 15); e[1] = new Employee("Harry Hacker", 50000, 1989, 10, 1); e[2] = new Employee("Tony Tester", 40000, 1990, 3, 15); try { PrintWriter out = new PrintWriter(new FileWriter("1.txt")); writeData(e, out); // out.close();********************************************************************** } catch (Exception e1) { e1.printStackTrace(); } System.out.println("*******是否要读取数据?********"); Scanner in1 = new Scanner(System.in); String yes = in1.nextLine(); if (yes.equalsIgnoreCase("YES")) { try { BufferedReader in = new BufferedReader(new FileReader("1.txt")); Employee[] result = readData(in); for (int i = 0; i < result.length; i++) System.out.println(result[i]); in.close(); } catch (Exception e2) { e2.printStackTrace(); } } } public static Employee[] readData(BufferedReader in) throws IOException { int length = Integer.parseInt(in.readLine()); Employee[] e = new Employee[length]; for (int i = 0; i < length; i++) { String line = in.readLine(); StringTokenizer token = new StringTokenizer(line, "|"); String name = token.nextToken(); double salary = Double.parseDouble(token.nextToken()); int year = Integer.parseInt(token.nextToken()); int month = Integer.parseInt(token.nextToken()); int day = Integer.parseInt(token.nextToken()); e[i] = new Employee(name, salary, year, month, day); } return e; } public static void writeData(Employee[] e, PrintWriter out) { out.println(e.length); for (int i = 0; i < e.length; i++) { String name = e[i].getName(); double salary = e[i].getSalary(); Date date = e[i].getHireDay(); Calendar c = new GregorianCalendar(); c.setTime(date); int year = c.get(Calendar.YEAR); int month = c.get(Calendar.MONTH) + 1; int day = c.get(Calendar.DAY_OF_MONTH); out.println(name + "|" + salary + "|" + year + "|" + month + "|" + day); } System.out.println("********写入数据完毕********"); }}class Employee { public Employee(String n, double s, int year, int month, int day) { name = n; salary = s; GregorianCalendar calendar = new GregorianCalendar(year, month - 1, day); hireDay = calendar.getTime(); } public String getName() { return name; } public double getSalary() { return salary; } public Date getHireDay() { return hireDay; } public void raiseSalary(double byPercent) { double raise = salary * byPercent / 100; salary += raise; } public String toString() { return getClass().getName() + "[name=" + name + ",salary=" + salary + ",hireDay=" + hireDay + "]"; } private String name; private double salary; private Date hireDay;}
结果是没有向文件写入任何数据,为什么呢?
唯一的错误就在main方法中没有调用out.close(),把数据从缓冲区刷新到文件。因此用完资源即时关闭是很重要的。