看了下Java Tutorials中的fork/join章節，整理下。
fork/join框架是ExecutorService接口的一個實現，可以幫助開發人員充分利用多核處理器的優勢，編寫出並行執行的程序，提高應用程序的性能；設計的目的是為了處理那些可以被遞歸拆分的任務。
fork/join框架與其它ExecutorService的實現類相似，會給線程池中的線程分發任務，不同之處在於它使用了工作竊取算法，所謂工作竊取，指的是對那些處理完自身任務的線程，會從其它線程竊取任務執行。
fork/join框架的核心是ForkJoinPool類，該類繼承了AbstractExecutorService類。ForkJoinPool實現了工作竊取算法並且能夠執行 ForkJoinTask任務。
在使用fork/join框架之前，我們需要先對任務進行分割，任務分割代碼應該跟下面的偽代碼類似：
if (任務足夠小){
直接執行該任務;
}else{
將任務一分為二;
執行這兩個任務並等待結果;
}
首先，我們會在ForkJoinTask的子類中封裝以上代碼，不過一般我們會使用更加具體的ForkJoinTask類型，如 RecursiveTask（可以返回一個結果）或RecursiveAction。
當寫好ForkJoinTask的子類後，創建該對象，該對象代表了所有需要完成的任務；然後將這個任務對象傳給ForkJoinPool實例的invoke()去執行即可。
為了更加直觀的理解fork/join框架是如何工作的，可以看一下下面這個例子。假定我們有一個圖像模糊的任務需要完成，原始圖像數據可以用一個整型數組表示，每一個整型元素包含了一個像素點的顏色值（RBG，存放在整型元素的不同位中）。目標圖像同樣是由一個整型數組構成，每個整型元素包含RBG顏色信息；
執行模糊操作需要遍歷原始圖像整型數組的每個元素，並對其周圍的像素點做均值操作（RGB均值），然後將結果存放到目標數組中。由於圖像是一個大數組，這個處理操作會花費一定的時間。但是有了fork/join框架，我們可以充分利用多核處理器進行並行計算。如下是一個可能的代碼實現（圖像做水平方向的模糊操作）：
Tips：該例子僅僅是闡述fork/join框架的使用，並不推薦使用該方法做圖像模糊，圖像邊緣處理也沒做判斷
public class ForkBlur extends RecursiveAction {
private static final long serialVersionUID = -8032915917030559798L;
private int[] mSource;
private int mStart;
private int mLength;
private int[] mDestination;
private int mBlurWidth = 15; // Processing window size, should be odd.
public ForkBlur(int[] src, int start, int length, int[] dst) {
mSource = src;
mStart = start;
mLength = length;
mDestination = dst;
}
// Average pixels from source, write results into destination.
protected void computeDirectly() {
int sidePixels = (mBlurWidth - 1) / 2;
for (int index = mStart; index < mStart + mLength; index++) {
// Calculate average.
float rt = 0, gt = 0, bt = 0;
for (int mi = -sidePixels; mi <= sidePixels; mi++) {
int mindex = Math.min(Math.max(mi + index, 0), mSource.length - 1);
int pixel = mSource[mindex];
rt += (float) ((pixel & 0x00ff0000) >> 16) / mBlurWidth;
gt += (float) ((pixel & 0x0000ff00) >> 8) / mBlurWidth;
bt += (float) ((pixel & 0x000000ff) >> 0) / mBlurWidth;
}
// Re-assemble destination pixel.
int dpixel = (0xff000000)
| (((int) rt) << 16)
| (((int) gt) << 8)
| (((int) bt) << 0);
mDestination[index] = dpixel;
}
}
...
現在，我們開始編寫compute()的實現方法，該方法分成兩部分：直接執行模糊操作和任務的劃分；一個數組長度阈值sThreshold可以幫助我們決定任務是直接執行還是進行劃分；
@Override
protected void compute() {
if (mLength < sThreshold) {
computeDirectly();
return;
}
int split = mLength / 2;
invokeAll(new ForkBlur(mSource, mStart, split, mDestination),
new ForkBlur(mSource, mStart + split, mLength - split,
mDestination));
}
接下來按如下步驟即可完成圖像模糊任務啦：
1、創建圖像模糊任務
ForkBlur fb = new ForkBlur(src, 0, src.length, dst);
2、創建ForkJoinPool
ForkJoinPool pool = new ForkJoinPool();
3、執行圖像模糊任務
pool.invoke(fb);
完整代碼如下：
/*
* Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
*
* - Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
*
* - Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* - Neither the name of Oracle or the names of its
* contributors may be used to endorse or promote products derived
* from this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
* IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
* THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
* LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
* NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
* SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
import java.awt.image.BufferedImage;
import java.io.File;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveAction;
import javax.imageio.ImageIO;
/**
* ForkBlur implements a simple horizontal image blur. It averages pixels in the
* source array and writes them to a destination array. The sThreshold value
* determines whether the blurring will be performed directly or split into two
* tasks.
*
* This is not the recommended way to blur images; it is only intended to
* illustrate the use of the Fork/Join framework.
*/
public class ForkBlur extends RecursiveAction {
private static final long serialVersionUID = -8032915917030559798L;
private int[] mSource;
private int mStart;
private int mLength;
private int[] mDestination;
private int mBlurWidth = 15; // Processing window size, should be odd.
public ForkBlur(int[] src, int start, int length, int[] dst) {
mSource = src;
mStart = start;
mLength = length;
mDestination = dst;
}
// Average pixels from source, write results into destination.
protected void computeDirectly() {
int sidePixels = (mBlurWidth - 1) / 2;
for (int index = mStart; index < mStart + mLength; index++) {
// Calculate average.
float rt = 0, gt = 0, bt = 0;
for (int mi = -sidePixels; mi <= sidePixels; mi++) {
int mindex = Math.min(Math.max(mi + index, 0), mSource.length - 1);
int pixel = mSource[mindex];
rt += (float) ((pixel & 0x00ff0000) >> 16) / mBlurWidth;
gt += (float) ((pixel & 0x0000ff00) >> 8) / mBlurWidth;
bt += (float) ((pixel & 0x000000ff) >> 0) / mBlurWidth;
}
// Re-assemble destination pixel.
int dpixel = (0xff000000)
| (((int) rt) << 16)
| (((int) gt) << 8)
| (((int) bt) << 0);
mDestination[index] = dpixel;
}
}
protected static int sThreshold = 10000;
@Override
protected void compute() {
if (mLength < sThreshold) {
computeDirectly();
return;
}
int split = mLength / 2;
invokeAll(new ForkBlur(mSource, mStart, split, mDestination),
new ForkBlur(mSource, mStart + split, mLength - split,
mDestination));
}
// Plumbing follows.
public static void main(String[] args) throws Exception {
String srcName = "C:\\test6.jpg";
File srcFile = new File(srcName);
BufferedImage image = ImageIO.read(srcFile);
System.out.println("Source image: " + srcName);
BufferedImage blurredImage = blur(image);
String dstName = "C:\\test6_out.jpg";
File dstFile = new File(dstName);
ImageIO.write(blurredImage, "jpg", dstFile);
System.out.println("Output image: " + dstName);
}
public static BufferedImage blur(BufferedImage srcImage) {
int w = srcImage.getWidth();
int h = srcImage.getHeight();
int[] src = srcImage.getRGB(0, 0, w, h, null, 0, w);
int[] dst = new int[src.length];
System.out.println("Array size is " + src.length);
System.out.println("Threshold is " + sThreshold);
int processors = Runtime.getRuntime().availableProcessors();
System.out.println(Integer.toString(processors) + " processor"
+ (processors != 1 ? "s are " : " is ")
+ "available");
ForkBlur fb = new ForkBlur(src, 0, src.length, dst);
ForkJoinPool pool = new ForkJoinPool();
long startTime = System.currentTimeMillis();
pool.invoke(fb);
long endTime = System.currentTimeMillis();
System.out.println("Image blur took " + (endTime - startTime) +
" milliseconds.");
BufferedImage dstImage =
new BufferedImage(w, h, BufferedImage.TYPE_INT_ARGB);
dstImage.setRGB(0, 0, w, h, dst, 0, w);
return dstImage;
}
}
測試了一下，執行效果如下：
除了我們上面提到的使用fork/join框架並行執行圖像模糊任務之外，在JAVA SE中，也已經利用fork/join框架實現了一些非常有用的特性。其中一個實現是在JAVA SE8 中java.util.Arrays 類的parallelSort()方法。這些方法和sort()方法類似，但是可以通過fork/join框架並行執行。對於大數組排序，在多核處理器系統中，使用並行排序方法比順序排序更加高效。當然，關於這些排序方法是如何利用fork/join框架不在本篇文章討論范圍，更多信息可以查看JAVA API文檔。
另一個fork/join框架的實現是在JAVA SE8中的java.util.streams包內，與Lambda表達式相關，更多信息，可以查看https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html鏈接。
