我知道已经太迟了，但我认为这里有更清晰的解决方案，更易于阅读……

/**
 * method converts {@link InputStream} Object into byte[] array.
 * 
 * @param stream the {@link InputStream} Object.
 * @return the byte[] array representation of received {@link InputStream} Object.
 * @throws IOException if an error occurs.
 */
public static byte[] streamToByteArray(InputStream stream) throws IOException {

    byte[] buffer = new byte[1024];
    ByteArrayOutputStream os = new ByteArrayOutputStream();

    int line = 0;
    // read bytes from stream, and store them in buffer
    while ((line = stream.read(buffer)) != -1) {
        // Writes bytes from byte array (buffer) into output stream.
        os.write(buffer, 0, line);
    }
    stream.close();
    os.flush();
    os.close();
    return os.toByteArray();
}

我试图编辑@numan的答案，修复了写垃圾数据，但编辑被拒绝。虽然这段简短的代码并不出色，但我看不到其他更好的答案。以下是我认为最有意义的建议:

ByteArrayOutputStream out = new ByteArrayOutputStream();
byte[] buffer = new byte[1024]; // you can configure the buffer size
int length;

while ((length = in.read(buffer)) != -1) out.write(buffer, 0, length); //copy streams
in.close(); // call this in a finally block

byte[] result = out.toByteArray();

ByteArrayOutputStream不需要关闭。为了可读性，省略了一些结构

下面是一个优化的版本，尽量避免复制数据字节:

private static byte[] loadStream (InputStream stream) throws IOException {
   int available = stream.available();
   int expectedSize = available > 0 ? available : -1;
   return loadStream(stream, expectedSize);
}

private static byte[] loadStream (InputStream stream, int expectedSize) throws IOException {
   int basicBufferSize = 0x4000;
   int initialBufferSize = (expectedSize >= 0) ? expectedSize : basicBufferSize;
   byte[] buf = new byte[initialBufferSize];
   int pos = 0;
   while (true) {
      if (pos == buf.length) {
         int readAhead = -1;
         if (pos == expectedSize) {
            readAhead = stream.read();       // test whether EOF is at expectedSize
            if (readAhead == -1) {
               return buf;
            }
         }
         int newBufferSize = Math.max(2 * buf.length, basicBufferSize);
         buf = Arrays.copyOf(buf, newBufferSize);
         if (readAhead != -1) {
            buf[pos++] = (byte)readAhead;
         }
      }
      int len = stream.read(buf, pos, buf.length - pos);
      if (len < 0) {
         return Arrays.copyOf(buf, pos);
      }
      pos += len;
   }
}

ByteArrayOutputStream out = new ByteArrayOutputStream();
byte[] buffer = new byte[1024];
while (true) {
    int r = in.read(buffer);
    if (r == -1) break;
    out.write(buffer, 0, r);
}

byte[] ret = out.toByteArray();

20年后，终于有了一个不需要第三方库的简单解决方案，这要感谢Java 9:

InputStream is;
…
byte[] array = is.readAllBytes();

还要注意方便的方法readNBytes(byte[] b, int off, int len)和transferTo(OutputStream)来解决重复的需求。

你可以使用Cactoos(我是它的开发者之一):

import org.cactoos.bytes.BytesOf;
byte[] array = new BytesOf(stream).asBytes();

你也可以将流转换为字符串:

import org.cactoos.text.TextOf;
String txt = new TextOf(stream).asString();

asBytes()和asString()方法都会抛出checked Exception。如果你不想捕获它，使用Unchecked*装饰器，例如:

import org.cactoos.bytes.BytesOf;
import org.cactoos.bytes.UncheckedBytes;
byte[] array = new UncheckedBytes(new BytesOf(stream)).asBytes();