import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;

public class Writer {


    public static void main(String args[]){
        doWrite("output.txt","Content to be appended to file");
    }

    public static void doWrite(String filePath,String contentToBeAppended){

       try(
            FileWriter fw = new FileWriter(filePath, true);
            BufferedWriter bw = new BufferedWriter(fw);
            PrintWriter out = new PrintWriter(bw)
          )
          {
            out.println(contentToBeAppended);
          }  
        catch( IOException e ){
        // File writing/opening failed at some stage.
        }

    }

}

您这样做是为了记录日志吗?如果是这样，有几个库。其中最流行的两个是Log4j和Logback。

Java 7 +

对于一次性任务，Files类可以很容易地做到这一点:

try {
    Files.write(Paths.get("myfile.txt"), "the text".getBytes(), StandardOpenOption.APPEND);
}catch (IOException e) {
    //exception handling left as an exercise for the reader
}

注意:如果文件不存在，上述方法将抛出NoSuchFileException。它也不会自动追加换行符(当追加到文本文件时，您经常希望这样)。另一种方法是同时传递CREATE和APPEND选项，如果文件不存在，则会先创建文件:

private void write(final String s) throws IOException {
    Files.writeString(
        Path.of(System.getProperty("java.io.tmpdir"), "filename.txt"),
        s + System.lineSeparator(),
        CREATE, APPEND
    );
}

但是，如果要多次写入同一个文件，那么上面的代码段必须多次打开和关闭磁盘上的文件，这是一个缓慢的操作。在这种情况下，BufferedWriter会更快:

try(FileWriter fw = new FileWriter("myfile.txt", true);
    BufferedWriter bw = new BufferedWriter(fw);
    PrintWriter out = new PrintWriter(bw))
{
    out.println("the text");
    //more code
    out.println("more text");
    //more code
} catch (IOException e) {
    //exception handling left as an exercise for the reader
}

注:

FileWriter构造函数的第二个参数将告诉它追加到文件中，而不是写入一个新文件。(如果该文件不存在，将创建该文件。) 对于昂贵的写入器(如FileWriter)，建议使用BufferedWriter。 使用PrintWriter可以访问您可能在System.out中习惯的println语法。 但是BufferedWriter和PrintWriter包装器并不是严格必要的。

老Java

try {
    PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("myfile.txt", true)));
    out.println("the text");
    out.close();
} catch (IOException e) {
    //exception handling left as an exercise for the reader
}

异常处理

如果你需要对旧Java进行健壮的异常处理，它会变得非常冗长:

FileWriter fw = null;
BufferedWriter bw = null;
PrintWriter out = null;
try {
    fw = new FileWriter("myfile.txt", true);
    bw = new BufferedWriter(fw);
    out = new PrintWriter(bw);
    out.println("the text");
    out.close();
} catch (IOException e) {
    //exception handling left as an exercise for the reader
}
finally {
    try {
        if(out != null)
            out.close();
    } catch (IOException e) {
        //exception handling left as an exercise for the reader
    }
    try {
        if(bw != null)
            bw.close();
    } catch (IOException e) {
        //exception handling left as an exercise for the reader
    }
    try {
        if(fw != null)
            fw.close();
    } catch (IOException e) {
        //exception handling left as an exercise for the reader
    }
}

你也可以试试这个:

JFileChooser c= new JFileChooser();
c.showOpenDialog(c);
File write_file = c.getSelectedFile();
String Content = "Writing into file"; //what u would like to append to the file



try 
{
    RandomAccessFile raf = new RandomAccessFile(write_file, "rw");
    long length = raf.length();
    //System.out.println(length);
    raf.setLength(length + 1); //+ (integer value) for spacing
    raf.seek(raf.length());
    raf.writeBytes(Content);
    raf.close();
} 
catch (Exception e) {
    //any exception handling method of ur choice
}

FileOutputStream stream = new FileOutputStream(path, true);
try {

    stream.write(

        string.getBytes("UTF-8") // Choose your encoding.

    );

} finally {
    stream.close();
}

然后在上游某处捕获IOException。

我的回答:

JFileChooser chooser= new JFileChooser();
chooser.showOpenDialog(chooser);
File file = chooser.getSelectedFile();
String Content = "What you want to append to file";

try 
{
    RandomAccessFile random = new RandomAccessFile(file, "rw");
    long length = random.length();
    random.setLength(length + 1);
    random.seek(random.length());
    random.writeBytes(Content);
    random.close();
} 
catch (Exception exception) {
    //exception handling
}

稍微扩展一下基普的回答， 下面是一个简单的Java 7+方法来追加一个新行到一个文件，如果它不存在就创建它:

try {
    final Path path = Paths.get("path/to/filename.txt");
    Files.write(path, Arrays.asList("New line to append"), StandardCharsets.UTF_8,
        Files.exists(path) ? StandardOpenOption.APPEND : StandardOpenOption.CREATE);
} catch (final IOException ioe) {
    // Add your own exception handling...
}

进一步指出:

The above uses the Files.write overload that writes lines of text to a file (i.e. similar to a println command). To just write text to the end (i.e. similar to a print command), an alternative Files.write overload can be used, passing in a byte array (e.g. "mytext".getBytes(StandardCharsets.UTF_8)). The CREATE option will only work if the specified directory already exists - if it doesn't, a NoSuchFileException is thrown. If required, the following code could be added after setting path to create the directory structure: Path pathParent = path.getParent(); if (!Files.exists(pathParent)) { Files.createDirectories(pathParent); }