Java流缓冲区问题
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;
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;
int actlen = in.read(b);
String str = new String(b);
System.out.println(str);
}
}
Java流缓冲区问题
</p>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。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;
e = new Employee("Carl Cracker", 75000, 1987, 12, 15);
e = new Employee("Harry Hacker", 50000, 1989, 10, 1);
e = 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);
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;
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 = 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.getName();
double salary = e.getSalary();
Date date = e.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。
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