[RESOLVIDO] Escrever na primeira linha em um arquivo .txt

try {

			FileOutputStream os = new FileOutputStream("arquivo.txt", true);
			OutputStreamWriter osw = new OutputStreamWriter(os);
			BufferedWriter bw = new BufferedWriter(osw);
			

			bw.append(messagem);

			bw.close();
				
		} catch (FileNotFoundException e1) {

			e1.printStackTrace();

		} catch (IOException e1) {

			e1.printStackTrace();
		}

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.OutputStream;
import java.io.OutputStreamWriter;


public class Teste {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
	
	    try {  
	    	String messagem  = "minha nova frase";
	    	File file = new File("arquivo.txt");
	    	StringBuilder conteudo = new StringBuilder();
	    	BufferedReader reader = new BufferedReader(new FileReader(file));
	    	String linha = null;
	    	boolean isPrimeiraLinha = true;
	    	while ((linha = reader.readLine()) != null) {
				if(isPrimeiraLinha){
					linha = messagem;
					isPrimeiraLinha = false;
				}
				conteudo.append(linha);
				conteudo.append("\n");
			}
	    	reader.close();
	    	BufferedWriter writer = new BufferedWriter(new FileWriter(file));  
	    	writer.write(conteudo.toString());
	    	writer.close();
	    	
	    } catch (FileNotFoundException e1) {  
	    	
	    	e1.printStackTrace();  
		     
	    } catch (IOException e1) {  
		     
		               e1.printStackTrace();  
		} 
	    

	}
}

try {
   FileWriter fw = new FileWriter("arquivo.txt", true);
   BufferedWriter bw = new BufferedWriter(fw);
   PrintWriter pw = new PrintWriter(bw);

   try {
      pw.println(mensagem); //Mensagem tem um "s" só.
   } finally {
      //Rode os fechamentos num finally para garantir que serão executados.
      if (pw != null) {
         pw.flush();
         pw.close();
      }
      if (bw != null) {
         bw.flush();
         bw.close();
      }
      if (fw != null) {
         fw.flush();
         fw.close();
      }
} catch (Exception e) {
   throw new RuntimeException(e);
}