使用 try-with-resources 不用手动close；推荐FileWriter，代码更加简洁。

package util.file;

import java.io.File;
import java.io.FileOutputStream;
import java.io.FileWriter;

public class StrWriteFileUtil {

    /**
     * FileWriter
     * 覆盖写入，不用判断文件是否存在
     *
     * @param str
     * @param path
     */
    public static void write(String str, String path) {

        try (FileWriter writer = new FileWriter(path)) {
            writer.write("");//清空原文件内容
            writer.write(str);
            writer.flush();
        } catch (Exception e) {
            e.printStackTrace();
        }

    }

    /**
     * FileWriter
     * 写入 追加文件
     *
     * @param str
     * @param path
     */
    public static void writeAppend(String str, String path) {

        try (FileWriter writer = new FileWriter(path, true)) {
            writer.write(str);
            writer.flush();
        } catch (Exception e) {
            e.printStackTrace();
        }

    }

    /**
     * FileOutputStream
     * 覆盖写入
     *
     * @param str
     * @param path
     */
    public static void outputStreamWrite(String str, String path) {

        File txt = new File(path);
        try (FileOutputStream fos = new FileOutputStream(txt)) {
            if (!txt.exists()) {
                txt.createNewFile();
            }
            byte[] bytes = str.getBytes();
            int b = bytes.length;   //是字节的长度，不是字符串的长度
            fos.write(bytes, 0, b);
        } catch (Exception e) {
            e.printStackTrace();

        }

    }

    /**
     * FileOutputStream
     * 写入 追加文件
     *
     * @param str
     * @param path
     */
    public static void outputStreamWriteAppend(String str, String path) {

        File txt = new File(path);
        try (FileOutputStream fos = new FileOutputStream(txt, true)) {
            if (!txt.exists()) {
                txt.createNewFile();
            }
            byte[] bytes = str.getBytes();
            int b = bytes.length;   //是字节的长度，不是字符串的长度
            fos.write(bytes, 0, b);
        } catch (Exception e) {
            e.printStackTrace();

        }

    }

    public static void main(String[] args) {
        String str = "hello world!";
        String path = "./test/3.txt";
        //write(str,path);
        //writeAppend(str,path);
        // outputStreamWrite(str, path);
        outputStreamWriteAppend(str, path);

    }

}