此代码适用于新的Java学习者：

private String textDataFromFile;

public String getFromFile(InputStream myInputStream) throws FileNotFoundException, IOException {

    BufferedReader bufferReader = new BufferedReader(new InputStreamReader(myInputStream));

    StringBuilder stringBuilder = new StringBuilder();

    String eachStringLine;

    while ((eachStringLine = bufferReader.readLine()) != null) {
        stringBuilder.append(eachStringLine).append("\n");
    }
    textDataFromFile = stringBuilder.toString();

    return textDataFromFile;
}

这里是在不使用任何第三方库的情况下将InputStream转换为String的完整方法。对于单线程环境使用StringBuilder，否则使用StringBuffer。

public static String getString( InputStream is) throws IOException {
    int ch;
    StringBuilder sb = new StringBuilder();
    while((ch = is.read()) != -1)
        sb.append((char)ch);
    return sb.toString();
}

如果不能使用Commons IO（FileUtils/IOUtils/CopyUtils），下面是一个使用BufferedReader逐行读取文件的示例：

public class StringFromFile {
    public static void main(String[] args) /*throws UnsupportedEncodingException*/ {
        InputStream is = StringFromFile.class.getResourceAsStream("file.txt");
        BufferedReader br = new BufferedReader(new InputStreamReader(is/*, "UTF-8"*/));
        final int CHARS_PER_PAGE = 5000; //counting spaces
        StringBuilder builder = new StringBuilder(CHARS_PER_PAGE);
        try {
            for(String line=br.readLine(); line!=null; line=br.readLine()) {
                builder.append(line);
                builder.append('\n');
            }
        } 
        catch (IOException ignore) { }

        String text = builder.toString();
        System.out.println(text);
    }
}

或者，如果你想要原始速度，我会根据Paul de Vrieze的建议（避免使用StringWriter（内部使用StringBuffer））提出一个变体：

public class StringFromFileFast {
    public static void main(String[] args) /*throws UnsupportedEncodingException*/ {
        InputStream is = StringFromFileFast.class.getResourceAsStream("file.txt");
        InputStreamReader input = new InputStreamReader(is/*, "UTF-8"*/);
        final int CHARS_PER_PAGE = 5000; //counting spaces
        final char[] buffer = new char[CHARS_PER_PAGE];
        StringBuilder output = new StringBuilder(CHARS_PER_PAGE);
        try {
            for(int read = input.read(buffer, 0, buffer.length);
                    read != -1;
                    read = input.read(buffer, 0, buffer.length)) {
                output.append(buffer, 0, read);
            }
        } catch (IOException ignore) { }

        String text = output.toString();
        System.out.println(text);
    }
}

考虑到文件，首先应该获得java.io.Reader实例。然后可以将其读取并添加到StringBuilder中（如果我们不在多个线程中访问StringBuffer，我们就不需要StringBuffer了，StringBuilder速度更快）。这里的技巧是我们在块中工作，因此不需要其他缓冲流。块大小被参数化以用于运行时性能优化。

public static String slurp(final InputStream is, final int bufferSize) {
    final char[] buffer = new char[bufferSize];
    final StringBuilder out = new StringBuilder();
    try (Reader in = new InputStreamReader(is, "UTF-8")) {
        for (;;) {
            int rsz = in.read(buffer, 0, buffer.length);
            if (rsz < 0)
                break;
            out.append(buffer, 0, rsz);
        }
    }
    catch (UnsupportedEncodingException ex) {
        /* ... */
    }
    catch (IOException ex) {
        /* ... */
    }
    return out.toString();
}

将inputStream转换为字符串的方法

public static String getStringFromInputStream(InputStream inputStream) {

    BufferedReader bufferedReader = null;
    StringBuilder stringBuilder = new StringBuilder();
    String line;

    try {
        bufferedReader = new BufferedReader(new InputStreamReader(
                inputStream));
        while ((line = bufferedReader.readLine()) != null) {
            stringBuilder.append(line);
        }
    } catch (IOException e) {
        logger.error(e.getMessage());
    } finally {
        if (bufferedReader != null) {
            try {
                bufferedReader.close();
            } catch (IOException e) {
                logger.error(e.getMessage());
            }
        }
    }
    return stringBuilder.toString();
}

InputStreamReader i = new InputStreamReader(s);
BufferedReader str = new BufferedReader(i);
String msg = str.readLine();
System.out.println(msg);

这是您的InputStream对象，它将被转换为String