1、使用String构造函数转换

byte[] bytes = new byte[0];
bytes = new byte[inputStream.available()];
inputStream.read(bytes);
String str = new String(bytes);

2、使用BufferedReader、collect和Collectors转换

String result = new BufferedReader(new InputStreamReader(inputStream))
        .lines().collect(Collectors.joining(System.lineSeparator()));

3、使用BufferedReader转换

InputStream inputStream = resource.getInputStream();
StringBuilder sb = new StringBuilder();
String line;
BufferedReader br = new BufferedReader(new InputStreamReader(inputStream));
while ((line = br.readLine()) != null) {
    sb.append(line);
}
String str = sb.toString();
System.out.println("输出"+str );

4、使用Scanner转换

Scanner s = new Scanner(inputStream).useDelimiter("\\A");
String str = s.hasNext() ? s.next() : "";

或者

String resource = new Scanner(inputStream).useDelimiter("\\Z").next();
return resource;

5、使用BufferedInputStream和ByteArrayOutputStream转换

BufferedInputStream bis = new BufferedInputStream(inputStream);
ByteArrayOutputStream buf = new ByteArrayOutputStream();
int result = bis.read();
while(result != -1) {
    buf.write((byte) result);
    result = bis.read();
}
String str = buf.toString();
return str;

6、使用Apache Common中IOUtils.toString()转换

String str = IOUtils.toString(inputStream, "utf-8");

或者

StringWriter writer = new StringWriter();
IOUtils.copy(inputStream, writer, StandardCharsets.UTF_8.name());
String str = writer.toString();

7、使用Google Guava中CharStreams.toString()转换

String str = CharStreams.toString(new InputStreamReader(inputStream, StandardCharsets.UTF_8));

相关文档：

Java 使用Stream I/O(InputStream/OutputStream)读写文件的方法及示例代码