InputSteam与String互相转化

在Java开发中，某些场景我们可能要将InputSteam与String互相转化。

InputStream转化为String

使用 InputStreamReader 和 StringBuilder`(JDK)

public class InputStream2String {
    public static void main(String[] args) {
        try {
            InputStream inputStream = new FileInputStream("E:/duckAndJava/IO/testFile.txt");    //路径修改为本地文件所在的位置

            char[] buffer = new char[1024];    //根据需要的数组大小进行自定义
            StringBuilder out = new StringBuilder();
            Reader in = new InputStreamReader(inputStream, "UTF-8");
            for (int numRead; (numRead = in.read(buffer, 0, buffer.length)) > 0; ) {
                out.append(buffer, 0, numRead);
            }
            String myString =  out.toString();

            System.out.println("myString = " + myString);

        }catch (IOException e){
            e.printStackTrace();
        }
    }
}

使用 inputStream.read() 和 StringBuilder

StringBuilder sb = new StringBuilder();
for (int ch; (ch = inputStream.read()) != -1; ) {
    sb.append((char) ch);
}
String myString = sb.toString();

使用 ByteArrayOutputStream 和 inputStream.read

ByteArrayOutputStream result = new ByteArrayOutputStream();
byte[] buffer = new byte[1024];
for (int length; (length = inputStream.read(buffer)) != -1; ) {
    result.write(buffer, 0, length);
}
String myString = result.toString("UTF-8");
ByteArrayOutputStream result = new ByteArrayOutputStream();
byte[] buffer = new byte[1024];
int length;
while ((length = inputStream.read(buffer)) != -1) {
    result.write(buffer, 0, length);
}
String str = result.toString(StandardCharsets.UTF_8.name());
return str;
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;

使用 BufferedInputStream 和 ByteArrayOutputStream

BufferedInputStream bis = new BufferedInputStream(inputStream);
ByteArrayOutputStream buf = new ByteArrayOutputStream();
for (int result = bis.read(); result != -1; result = bis.read()) {
    buf.write((byte) result);
}
String myString = buf.toString("UTF-8");

使用 BufferedReader

String newLine = System.getProperty("line.separator");
BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
StringBuilder result = new StringBuilder();
for (String line; (line = reader.readLine()) != null; ) {
    if (result.length() > 0) {
        result.append(newLine);
    }
    result.append(line);
}
String myString = result.toString();
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();
return str;
String result = new BufferedReader(new InputStreamReader(inputStream))
        .lines().collect(Collectors.joining(System.lineSeparator()));
String result = new BufferedReader(new InputStreamReader(inputStream))
       .lines().parallel().collect(Collectors.joining(System.lineSeparator()));

使用 Stream API 或 parallel Stream API

String myString = new BufferedReader(new InputStreamReader(inputStream)).lines().collect(Collectors.joining("\n"));

或

String myString = new BufferedReader(new InputStreamReader(inputStream)).lines().parallel().collect(Collectors.joining("\n"));

使用 StringWriter 和 IOUtils.copy (Apache Commons)

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

甚至可以直接这样用

String result = IOUtils.toString(inputStream, StandardCharsets.UTF_8);

使用 CharStreams (Google Guava)

String result = CharStreams.toString(new InputStreamReader(inputStream, Charsets.UTF_8));

String str = new String(ByteStreams.toByteArray(inputStream))

分别按照字符串长度来进行测试。

当使用的是一个小字符串（length=175），得到的性能测试结果如下：

当使用的是一个长字符串（length=50100），得到的性能测试结果如下：

为了更加直观，按照字符串的长度与相应函数消耗的平均时间，做了如下的表格：

更加直观的表格图，如下：

JDK原生提供

byte[] bytes = new byte[0];
bytes = new byte[inputStream.available()];
inputStream.read(bytes);
String str = new String(bytes);
Scanner s = new Scanner(inputStream).useDelimiter("\\A");
String str = s.hasNext() ? s.next() : "";
String resource = new Scanner(inputStream).useDelimiter("\\Z").next();
return resource;

String转化为InputStream

JDK原生提供

InputStream is = new ByteArrayInputStream(str.getBytes());

Apache Common提供

InputStream targetStream = IOUtils.toInputStream(str, StandardCharsets.UTF_8.name());

Google Guava提供

InputStream targetStream =
        new ReaderInputStream(CharSource.wrap(str).openStream(), StandardCharsets.UTF_8.name());