InputStream IS=new URL("http://www.petrol.si/api/gas_prices.json").openStream();   

  ByteArrayOutputStream BAOS=new ByteArrayOutputStream();
  IOUtils.copy(IS, BAOS);
  String d= new String(BAOS.toByteArray(),"UTF-8");           

System.out.println(d);

这是一个改编自org.apache.commons.io.IOUtils源代码的答案，适用于那些想要apache实现但不想要整个库的人。

private static final int BUFFER_SIZE = 4 * 1024;

public static String inputStreamToString(InputStream inputStream, String charsetName)
        throws IOException {
    StringBuilder builder = new StringBuilder();
    InputStreamReader reader = new InputStreamReader(inputStream, charsetName);
    char[] buffer = new char[BUFFER_SIZE];
    int length;
    while ((length = reader.read(buffer)) != -1) {
        builder.append(buffer, 0, length);
    }
    return builder.toString();
}

另一个，对于所有Spring用户：

import java.nio.charset.StandardCharsets;
import org.springframework.util.FileCopyUtils;

public String convertStreamToString(InputStream is) throws IOException { 
    return new String(FileCopyUtils.copyToByteArray(is), StandardCharsets.UTF_8);
}

org.springframework.util.StreamUtils中的实用程序方法与FileCopyUtils中类似，但它们在完成后会使流保持打开状态。

JDK 7/8应答，关闭流并仍然抛出IOException：

StringBuilder build = new StringBuilder();
byte[] buf = new byte[1024];
int length;
try (InputStream is = getInputStream()) {
  while ((length = is.read(buf)) != -1) {
    build.append(new String(buf, 0, length));
  }
}

如果使用流读取器，请确保在结束时关闭流

private String readStream(InputStream iStream) throws IOException {
    //build a Stream Reader, it can read char by char
    InputStreamReader iStreamReader = new InputStreamReader(iStream);
    //build a buffered Reader, so that i can read whole line at once
    BufferedReader bReader = new BufferedReader(iStreamReader);
    String line = null;
    StringBuilder builder = new StringBuilder();
    while((line = bReader.readLine()) != null) {  //Read till end
        builder.append(line);
        builder.append("\n"); // append new line to preserve lines
    }
    bReader.close();         //close all opened stuff
    iStreamReader.close();
    //iStream.close(); //EDIT: Let the creator of the stream close it!
                       // some readers may auto close the inner stream
    return builder.toString();
}

编辑：在JDK7+上，可以使用trywithresources构造。

/**
 * Reads the stream into a string
 * @param iStream the input stream
 * @return the string read from the stream
 * @throws IOException when an IO error occurs
 */
private String readStream(InputStream iStream) throws IOException {

    //Buffered reader allows us to read line by line
    try (BufferedReader bReader =
                 new BufferedReader(new InputStreamReader(iStream))){
        StringBuilder builder = new StringBuilder();
        String line;
        while((line = bReader.readLine()) != null) {  //Read till end
            builder.append(line);
            builder.append("\n"); // append new line to preserve lines
        }
        return builder.toString();
    }
}

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();
}