Đọc ghi tệp (file) trong java

1. ByteArrayOutputStream

InputStream in = context.openFileInput("datafile.txt");
ByteArrayOutputStream bais = new ByteArrayOutputStream();
byte[] bytes = new byte[8192];
for(int len; (lne = in.read(bytes) > 0;)
   bais.write(bytes, 0, len);
in.close();
return bais.toByteArray();

2. FileOutputStream

public static void main(String[] args) throws Exception
{
    byte[] array = new byte[1024*4];
    FileInputStream fis = new FileInputStream(new File("<Path-to-input-file>"));
    FileOutputStream fos = new FileOutputStream(new File("<Path-to-output-file>"));
    int length = 0;
    while((length = fis.read(array)) != -1)
    {
        fos.write(array, 0, length);
    }
    fis.close();
    fos.close();
}

3. Use FileChannel 

4. Apache Commons IO FileUtils. 
FileUtils class
avoid DOS attack từ tầng trình diễn
String content =FileUtils.readFileToString(new File(filePath));

5. Delete temporary files

 private void deleteCompressTempFiles(HttpServletRequest request) {

        String path = request.getSession().getServletContext().getRealPath("/");

        //File file = File.createTempFile("getter", ".html", new File(path + "/tmp/"));

        //File dir = new File(path + "/temp/");

        File dir = new File(System.getProperty("java.io.tmpdir"));

        File[] files = dir.listFiles(new FilenameFilter() {

            public boolean accept(File dir, String name) {

                String tmpName = name.toLowerCase();

                return tmpName.startsWith("poi-sxssf-sheet") && (tmpName.endsWith(".xml") || tmpName.endsWith(".gz"));

            }

        });

        if (files != null) {

            for (File file : files) {

                file.delete();

            }

        }

    }

//Absolute file path on server
String serverFileName = updateForm.getServerFileName();
String tempPath = ResourceBundleUtils.getResource("UPLOAD_FILE_PATH");
String serverFilePath = req.getSession().getServletContext().getRealPath(tempPath); //server
String fullServerFileName = tempPath + serverFileName;
File clientFile = new File(fullServerFileName);

Tham khảo:

File to byte[] in Java

Best Way to Append Data to File in Java

JAVA HEAP SIZE and files to byte[]-Array

How to Read a Large File Efficiently with Java

Apache commons-io