java - 这个 FileWriter 是如何工作的？

Question

这是一个代码:

import java.io.*;

public class FileCharCopier
{
  public static void main(String args[]) throws IOException
  {

    File f1=new File("scjp.txt");
    File f2=new File("scjpcopy.txt");

    FileReader in=new FileReader(f1);
    FileWriter out=new FileWriter(f2);

    int c;

    while((c=in.read())!=1)
            {
      out.write(c);
      in.close();
      out.flush();
      out.close();


  }
}
}

我在同一目录中同时拥有 scjp 和 scjpcopy.txt

但是当我运行这个程序时，我得到了这些错误:

java.io.IOException: Stream closed
    at sun.nio.cs.StreamEncoder.ensureOpen(Unknown Source)
    at sun.nio.cs.StreamEncoder.write(Unknown Source)
    at sun.nio.cs.StreamEncoder.write(Unknown Source)
    at java.io.OutputStreamWriter.write(Unknown Source)
    at FileCharCopier.main(FileCharCopier.java:18)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:272)

第18行指的是

out.write(c);

谁能改正错误？

Answer 1

基本上，您将在 while-loop 的第一次迭代中关闭输入和输出流。这意味着当您第二次尝试循环时，流将关闭并抛出异常。

相反，例如，您应该在使用完流后关闭它们。

public class FileCharCopier {

    public static void main(String args[]) throws IOException {

        File f1 = new File("scjp.txt");
        File f2 = new File("scjpcopy.txt");

        FileReader in = null;
        FileWriter out = null;

        try {

            in = new FileReader(f1);
            out = new FileWriter(f2);

            int c;

            while ((c = in.read()) != 1) {
                out.write(c);
            }

            out.flush();

        } finally {
            try {
                out.close();
            } catch (Exception e) {
            }
            try {
                in.close();
            } catch (Exception e) {
            }
        }
    }
}

这使用 try-finally block 来确保在存在循环或发生某些错误时尽最大努力关闭流。

现在，如果您有幸使用 Java 7，则可以改用 try-with-resources 语句，例如...

public class FileCharCopier {

    public static void main(String args[]) throws IOException {

        File f1 = new File("scjp.txt");
        File f2 = new File("scjpcopy.txt");

        try (FileReader in = new FileReader(f1); FileWriter out = new FileWriter(f2)) {

            int c;

            while ((c = in.read()) != 1) {
                out.write(c);
            }

            out.flush();

        } finally {
        }
    }
}

看看The try-with-resources statement了解更多详情