Very slow download speed using Java in Raspberry Pi 3

Question

I wrote the following code in Java for downloading a file in a Raspberry Pi 3:

String fileUrl = "...";
URL urlObj = new URL(fileUrl);
HttpURLConnection con = (HttpURLConnection) urlObj.openConnection();
InputStream in = con.getInputStream();

byte[] buffer = new byte[8*1024];
long t = System.nanoTime();
int read;

while ((read = in.read(buffer)) != -1) {
    System.out.println("Read " + read + "B in " + (System.nanoTime() - t)/1000000.0 + " ms");
    t = System.nanoTime();
}

Even though I am using an 8 KB buffer, the average download speed is 1389 B in around 205 ms, which translates to 6.78 KB/s:

Download speed measurements

I also noticed that the CPU usage while executing this code is always 25%. Since the RPi's CPU has 4 cores, I asume it is using 100% of a single core. I know this is a weak processor, but downloading a file isn't a demanding task, so this odd behaviour puzzles me.

Answer 1

I finally solved the problem!

I removed OpenJDK and installed Oracle JDK:

sudo apt-get purge openjdk-8-jdk
sudo apt-get purge openjdk-8-jre
sudo apt-get autoremove
sudo apt-get install oracle-java8-jdk

If Oracle's JDK was correctly installed, running java -version should read something like this:

java version "1.8.0_65"
Java(TM) SE Runtime Environment (build 1.8.0_65-b17)
Java HotSpot(TM) Client VM (build 25.65-b01, mixed mode)

With this, download speeds went from around 5 KB/s to around 450 KB/s, which I expected from my 4 Mbps connection.

I reinstalled OpenJDK out of curiosity and the download speed was slow again, so this jdk was the problem, as Erich Kitzmueller and pvg suggested.

Thanks to everyone for your suggestions!