Java调试NullPointerException

Question

我有这个java代码，并在运行它时捕获了一个nullPointerException。通过使用eclipse进行调试，我发现msiLine参数为null，但无法找到原因。

                String msiAbsolutePath = msiFiles.getAbsolutePath();
                String filePath = msiAbsolutePath.substring(0,msiAbsolutePath.lastIndexOf(File.separator));
                new File(filePath+"/Emboss").mkdir();
                new File(filePath+"/Report").mkdir();
                File files = sort(msiFiles,filePath);

                BufferedReader msiReader = new BufferedReader(new FileReader(files));
                BufferedReader embReader = new BufferedReader(new FileReader(embFiles));
                String[] msiTokens = msiFiles.getName().split("\\.(?=[^\\.]+$)");
                String[] embTokens = embFiles.getName().split("\\.(?=[^\\.]+$)");
                final String msiFileName = msiTokens[0];
                final String embFileName = embTokens[0];
                Date date = new Date();
                DateFormat dateFormatName = new SimpleDateFormat("MMdd");
                PrintWriter msiWrite = new PrintWriter(new BufferedWriter(new FileWriter(filePath+"/Emboss/"+msiFileName+".PRN")));
                PrintWriter embWrite = new PrintWriter(new BufferedWriter(new FileWriter(filePath+"/Emboss/"+embFileName+".PRN")));
                PrintWriter reportWrite = new PrintWriter(new BufferedWriter(new FileWriter(filePath+"/Report/CS"+dateFormatName.format(date)+".RPT")));


                cnt=totalLines=0;
                //String msiLine = msiReader.readLine();
                String msiLine="";
                String embLine = embReader.readLine();
        here>>> for(msiLine = msiReader.readLine(); msiLine.length() >= 60; msiLine = msiReader.readLine())
                {
                    embLine = embReader.readLine();
                    msiWrite.print();
                    :
                    :

                    }
                    statMessage = "Completed " + current + " out of " + lengthOfTask + ".";
                }
                :
                :
            }catch(IllegalStateException | IOException | NullPointerException d){
                d.printStackTrace();
            }
        }
    }

    File sort(File file, String filepath){
        File files = new File(filepath + "\\tempMSIfile.msi");
        try{

            BufferedReader reader = new BufferedReader(new FileReader(file));
            Map<String, String> map=new TreeMap<String, String>();
            String line=reader.readLine();
            for(line = reader.readLine(); line.length() >= 60; line = reader.readLine()){
                map.put(getField(line),line);
            }
            reader.close();
            PrintWriter writer = new PrintWriter(new BufferedWriter(new FileWriter(files)));
            for(String val : map.values()){
                writer.print("" +val);  
                writer.print(NEWLINE);
            }
            writer.close();

        }catch(IllegalStateException | IOException | NullPointerException d){
            LOG.fine("Error, " + d.getMessage());
        }
        return files;
    }

    String getField(String line) {
        return line.substring(10, 26);//extract value you want to sort on
    }

有人能解释一下为什么msiLine是空的吗？

我认为这涉及到File files = sort(msiFiles,filePath);，但是eclipse报告说files参数确实有正确的文件/东西(？)分配给它。

如果有人想知道，tempMSIfile.msi不是空的。

Answer 1

当到达输入结束时，Reader的.readLine()返回null。因此，您的条件：

msiLine.length() < 60`

可以触发NullPointerException。将其更改为在测试长度之前添加一个空测试。

编辑：@KubaSpatny指出，将条件更改为：

msiLine != null && msiLine.length() < 60

或者，正如您的评论所建议的那样，将msiLine != null作为条件，并将其作为循环中的第一个指令：

if (msiLine.length() < 60)
    continue; // or break;, depending on what you want to do

您的sort()方法也面临同样的问题。

Answer 2

阅读文献资料 for readLine()

Returns: A String containing the contents of the line, not including any line-termination characters, or null if the end of the stream has been reached

这意味着:如果msiLine是循环中的null，则文件已被完全读取。在这种情况下，退出循环：

if(msiLine==null){
   break;
}

或者将检查添加到循环头中。

Answer 3

java.io.BufferedReader#readLine()的Javadoc声明它返回

 A String containing the contents of the line, not including
 any line-termination characters, or null if the end of the
 stream has been reached

您可能遇到了文件的末尾。当您到达文件末尾时，为msiLine添加一个空检查以检查。