/
ParallelCountBytes.java
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/
ParallelCountBytes.java
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/*
* Copyright 2016 Google LLC
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.google.cloud.examples.nio;
import com.google.common.base.Stopwatch;
import com.google.common.io.BaseEncoding;
import java.io.Closeable;
import java.io.IOException;
import java.net.URI;
import java.nio.ByteBuffer;
import java.nio.channels.SeekableByteChannel;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.security.MessageDigest;
import java.util.ArrayDeque;
import java.util.Queue;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
/**
* ParallelCountBytes will read through the whole file given as input.
*
* <p>This example shows how to go through all the contents of a file, in order, using multithreaded
* NIO reads. It prints a MD5 hash and reports how long it took.
*
* <p>See the <a
* href="https://github.com/googleapis/google-cloud-java/blob/master/google-cloud-examples/README.md">
* README</a> for compilation instructions. Run this code with
*
* <pre>{@code target/appassembler/bin/ParallelCountBytes <file>}</pre>
*/
public class ParallelCountBytes {
/**
* WorkUnit holds a buffer and the instructions for what to put in it.
*
* <p>Use it like this:
*
* <ol>
* <li>call()
* <li>the data is now in buf, you can access it directly
* <li>if need more, call resetForIndex(...) and go back to the top.
* <li>else, call close()
* </ol>
*/
private static class WorkUnit implements Callable<WorkUnit>, Closeable {
public final ByteBuffer buf;
final SeekableByteChannel chan;
final int blockSize;
int blockIndex;
public WorkUnit(SeekableByteChannel chan, int blockSize, int blockIndex) {
this.chan = chan;
this.buf = ByteBuffer.allocate(blockSize);
this.blockSize = blockSize;
this.blockIndex = blockIndex;
}
@Override
public WorkUnit call() throws IOException {
long pos = ((long) blockSize) * blockIndex;
if (pos > chan.size()) {
return this;
}
chan.position(pos);
// read until buffer is full, or EOF
while (chan.read(buf) > 0) {}
;
return this;
}
public WorkUnit resetForIndex(int blockIndex) {
this.blockIndex = blockIndex;
buf.flip();
return this;
}
public void close() throws IOException {
chan.close();
}
}
/** See the class documentation. */
public static void main(String[] args) throws Exception {
if (args.length == 0 || args[0].equals("--help")) {
help();
return;
}
for (String a : args) {
countFile(a);
}
}
/**
* Print the length and MD5 of the indicated file.
*
* <p>This uses the normal Java NIO Api, so it can take advantage of any installed NIO Filesystem
* provider without any extra effort.
*/
private static void countFile(String fname) throws Exception {
// large buffers pay off
final int bufSize = 50 * 1024 * 1024;
Queue<Future<WorkUnit>> work = new ArrayDeque<>();
Path path = Paths.get(new URI(fname));
long size = Files.size(path);
System.out.println(fname + ": " + size + " bytes.");
int nThreads = (int) Math.ceil(size / (double) bufSize);
if (nThreads > 4) nThreads = 4;
System.out.println("Reading the whole file using " + nThreads + " threads...");
Stopwatch sw = Stopwatch.createStarted();
long total = 0;
MessageDigest md = MessageDigest.getInstance("MD5");
ExecutorService exec = Executors.newFixedThreadPool(nThreads);
int blockIndex;
for (blockIndex = 0; blockIndex < nThreads; blockIndex++) {
work.add(exec.submit(new WorkUnit(Files.newByteChannel(path), bufSize, blockIndex)));
}
while (!work.isEmpty()) {
WorkUnit full = work.remove().get();
md.update(full.buf.array(), 0, full.buf.position());
total += full.buf.position();
if (full.buf.hasRemaining()) {
full.close();
} else {
work.add(exec.submit(full.resetForIndex(blockIndex++)));
}
}
exec.shutdown();
long elapsed = sw.elapsed(TimeUnit.SECONDS);
System.out.println("Read all " + total + " bytes in " + elapsed + "s. ");
String hex = String.valueOf(BaseEncoding.base16().encode(md.digest()));
System.out.println("The MD5 is: 0x" + hex);
if (total != size) {
System.out.println(
"Wait, this doesn't match! We saw "
+ total
+ " bytes, "
+ "yet the file size is listed at "
+ size
+ " bytes.");
}
}
private static void help() {
String[] help = {"The argument is a <path>", "and we show the length of that file."};
for (String s : help) {
System.out.println(s);
}
}
}