重复写入文件

Question

我是一个中等经验的 C++ 人，正在慢慢学习 Java。我正在编写一个需要执行以下操作的程序：

• 创建一个简单的文本文件，默认目录就可以了
• 在程序运行时，定期将一行数据写入文件。根据许多因素，程序可能会写入文件一次或一百万次。无法知道哪个写入将是最后一个。

我一直在研究不同的方法来做到这一点，这是我想出的工作代码。有两个文件，“PeteProgram.java”和“PeteFileMgr.java”：

/*
"PeteProgram.java"
*/

import java.io.*;
import java.lang.String;

public class PeteProgram {

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

      String PeteFilename="MyRecordsFile.txt";
      Writer writer = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(PeteFilename), "utf-8"));
      PeteFileMgr MyPeteFileMgr = new PeteFileMgr(writer);

      MyPeteFileMgr.AddThisString(writer, "Add this line #1\n");
      MyPeteFileMgr.AddThisString(writer, "Add this line #2\n");
      MyPeteFileMgr.AddThisString(writer, "Add this line #3\n");

    }
}


//=====================================================================================================
//=====================================================================================================


/*
"PeteFileMgr.java"
*/

import java.io.*;

public class PeteFileMgr {

  public PeteFileMgr(Writer writer) {

       try {
          writer.write("File created!");

             } catch (IOException ex) {
          // report
          } finally {
             try {writer.close();} catch (Exception ex) {}
          }
      }

  void AddThisString(Writer writer, String AddThis) {

    try {
          writer.append(AddThis);

             } catch (IOException ex) {
          // report
          } finally {
             try {writer.close();} catch (Exception ex) {}
          }
      } 
  }   

文件的初始创建工作正常。但是，要添加的行不会写入文件中。因为程序编译和运行没有错误，我假设程序尝试编写添加的行，失败并抛出异常。 （不幸的是，我正在使用原始编译器/调试器，无法查看是否是这种情况。）

有人发现我的错误吗？

       Many thanks!
            -P

Answer 1

那是因为您没有刷新Writer。您应该不时致电flush。此外，您应该在应用结束时close@Writer，而不是在将内容写入其中之后。 close 方法自动刷新 writer 的内容。

所以，你的代码应该是这样的：

public class PeteProgram {
    public static void main(String[] args) {
        String peteFilename = "MyRecordsFile.txt";
        //here's when the physical file is created
        Writer writer = null;
        try {
            writer = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(peteFilename), "utf-8"));
        PeteFileMgr peteFileMgr = new PeteFileMgr(writer);

        peteFileMgr.addThisString(writer, "Add this line #1\n");
        peteFileMgr.addThisString(writer, "Add this line #2\n");
        peteFileMgr.addThisString(writer, "Add this line #3\n");
        } catch (IOException e) {
            //handle the exception
            //basic handling
            e.printStacktrace();
        } finally {
            //this is a must!
            try { writer.close(); } catch(IOException silent) { }
        }
    }
}

public class PeteFileMgr {
    public PeteFileMgr(Writer writer) {
        try {
            //this method is not creating the physical file
            writer.write("File created!");
        } catch (IOException ex) {
          // report
        } finally {
            //remove this call to close
            //try {writer.close();} catch (Exception ex) {}
        }
    }

    public void addThisString(Writer writer, String addThis) {
        try {
            writer.append(addThis);
        } catch (IOException ex) {
            // report
        } finally {
            //remove this call to close
            //try {writer.close();} catch (Exception ex) {}
        }
    }
}

或者，如果使用 Java 7 或更高版本，则使用 try-with-resources：

public class PeteProgram {
    public static void main(String[] args) {
        String peteFilename = "MyRecordsFile.txt";
        //here's when the physical file is created
        try (Writer writer = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(peteFilename), "utf-8"))) {
        PeteFileMgr peteFileMgr = new PeteFileMgr(writer);

        peteFileMgr.addThisString(writer, "Add this line #1\n");
        peteFileMgr.addThisString(writer, "Add this line #2\n");
        peteFileMgr.addThisString(writer, "Add this line #3\n");
        } catch (IOException e) {
            //handle the exception
            //basic handling
            e.printStacktrace();
        }
    }
}