这段代码将满足您的需要:

   FileWriter fw=new FileWriter("C:\\file.json",true);
   fw.write("ssssss");
   fw.close();

java 7 +

在我看来，因为我是普通java的粉丝，我会建议它是上述答案的组合。也许我派对迟到了。代码如下:

 String sampleText = "test" +  System.getProperty("line.separator");
 Files.write(Paths.get(filePath), sampleText.getBytes(StandardCharsets.UTF_8), 
 StandardOpenOption.CREATE, StandardOpenOption.APPEND);

如果文件不存在，则创建该文件;如果文件已经存在，则追加 sampleText到现有文件。使用它，可以避免向类路径添加不必要的库。

使用java.nio.Files和java.nio.file.StandardOpenOption

    PrintWriter out = null;
    BufferedWriter bufWriter;

    try{
        bufWriter =
            Files.newBufferedWriter(
                Paths.get("log.txt"),
                Charset.forName("UTF8"),
                StandardOpenOption.WRITE, 
                StandardOpenOption.APPEND,
                StandardOpenOption.CREATE);
        out = new PrintWriter(bufWriter, true);
    }catch(IOException e){
        //Oh, no! Failed to create PrintWriter
    }

    //After successful creation of PrintWriter
    out.println("Text to be appended");

    //After done writing, remember to close!
    out.close();

这将使用Files创建一个BufferedWriter，它接受StandardOpenOption参数，并从结果BufferedWriter创建一个自动刷新的PrintWriter。然后可以调用PrintWriter的println()方法来写入文件。

这段代码中使用的StandardOpenOption参数:打开文件进行写入，仅追加到文件中，并在文件不存在时创建文件。

路径。get("path here")可以用new File("path here"). topath()代替。 和字符集。forName(“字符集名称”)可以修改以适应所需的字符集。

在Java-7中也可以这样做:

import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;

//---------------------

Path filePath = Paths.get("someFile.txt");
if (!Files.exists(filePath)) {
    Files.createFile(filePath);
}
Files.write(filePath, "Text to be added".getBytes(), StandardOpenOption.APPEND);

1.7的方法:

void appendToFile(String filePath, String content) throws IOException{

    Path path = Paths.get(filePath);

    try (BufferedWriter writer = 
            Files.newBufferedWriter(path, 
                    StandardOpenOption.APPEND)) {
        writer.newLine();
        writer.append(content);
    }

    /*
    //Alternative:
    try (BufferedWriter bWriter = 
            Files.newBufferedWriter(path, 
                    StandardOpenOption.WRITE, StandardOpenOption.APPEND);
            PrintWriter pWriter = new PrintWriter(bWriter)
            ) {
        pWriter.println();//to have println() style instead of newLine();   
        pWriter.append(content);//Also, bWriter.append(content);
    }*/
}

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。