聽著張孝祥老師關于緩沖區知識的課，發現還是有一些沒有掌握，動手試了一下，果然發現了問題。
先講一下關于java緩沖區的知識，應用程序和IO設備之間存在一個緩沖區，一般流是沒有緩沖區的，但是如果存在緩沖區，就會發現很大的問題。
錯誤代碼如下：為了確保問題發生，我使用了BufferedOutputStream，使得手動構造出了一個緩沖區。
- 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);
-
- }
- }
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發現什么問題了嗎？
因為如果沒有緩沖區，應用程序每次IO都要和設備進行通信，效率很低，因此緩沖區為了提高效率，當寫入設備時，先寫入緩沖區，等到緩沖區有足夠多的數據時，就整體寫入設備。這就是問題所在，上個例子中，當我們寫入hello時，由于hello占用空間很小，所以暫時存放在緩沖區中，后來輸入流想要從文件中讀取，但是由于文件中沒有字節，所以不能讀取hello。
這里，解決方法很簡單，只要調用out.flush() 或者out.close()即可，這是把緩沖區的數據手動寫入文件。
正確代碼如下：
- 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);
-
- }
- }
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接下來又是我遇到的一個例子，這個例子也很明顯的反應出緩沖區的問題。
- 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("*******是否要讀取數據？********");
- 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("********寫入數據完畢********");
- }
-
- }
-
- 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;
- }
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結果是沒有向文件寫入任何數據，為什么呢？
唯一的錯誤就在main方法中沒有調用out.close()，把數據從緩沖區刷新到文件。因此用完資源即時關閉是很重要的。