我有一个多线程Java应用程序，它将图像分成4个块，然后4个线程（我有一个四核CPU）分别处理图像的每个块，将其转换为灰度。
我发现它的速度相当慢，因此我使用了NetBeans分析器，并发现这些线程在相当长的时间内处于“监视”（等待）状态。例如，
（绿色=运行，红色=监视）
我尝试使用不同数量的线程，例如 2，发现仍然会出现这种情况（唯一不会出现此问题的是使用1个线程时）。
在线程内部，我注释了它们代码的一些部分，直到我将“大延迟”缩小到以下语句：
newImage.setRGB(i,j,newColor.getRGB()); // Write the new value for that pixel
如果将此行注释掉，代码运行速度会快得多（几乎快5倍），而且没有线程监控：
那么为什么这一行会引起如此大的延迟呢？是Color库（与BufferedImage一起）吗？现在我要尝试获取一个int数组作为RGB值，而不是使用Color对象，看看效果如何。以下是源代码：
PixelsManipulation.java（主类）：
public final class PixelsManipulation{
private static Sequential sequentialGrayscaler = new Sequential();
public static void main(String[] args) throws FileNotFoundException, IOException, InterruptedException {
File file = new File("src/pixelsmanipulation/hiresimage.jpg");
FileInputStream fis = new FileInputStream(file);
BufferedImage image = ImageIO.read(fis); //reading the image file
int rows = 2; // 2 rows and 2 cols will split the image into quarters
int cols = 2;
int chunks = rows * cols; // 4 chunks, one for each quarter of the image
int chunkWidth = image.getWidth() / cols; // determines the chunk width and height
int chunkHeight = image.getHeight() / rows;
int count = 0;
BufferedImage imgs[] = new BufferedImage[chunks]; // Array to hold image chunks
for (int x = 0; x < rows; x++) {
for (int y = 0; y < cols; y++) {
//Initialize the image array with image chunks
imgs[count] = new BufferedImage(chunkWidth, chunkHeight, image.getType());
// draws the image chunk
Graphics2D gr = imgs[count++].createGraphics(); // Actually create an image for us to use
gr.drawImage(image, 0, 0, chunkWidth, chunkHeight, chunkWidth * y, chunkHeight * x, chunkWidth * y + chunkWidth, chunkHeight * x + chunkHeight, null);
gr.dispose();
}
}
//writing mini images into image files
for (int i = 0; i < imgs.length; i++) {
ImageIO.write(imgs[i], "jpg", new File("img" + i + ".jpg"));
}
System.out.println("Mini images created");
// Start threads with their respective quarters (chunks) of the image to work on
// I have a quad-core machine, so I can only use 4 threads on my CPU
Parallel parallelGrayscaler = new Parallel("thread-1", imgs[0]);
Parallel parallelGrayscaler2 = new Parallel("thread-2", imgs[1]);
Parallel parallelGrayscaler3 = new Parallel("thread-3", imgs[2]);
Parallel parallelGrayscaler4 = new Parallel("thread-4", imgs[3]);
// Sequential:
long startTime = System.currentTimeMillis();
sequentialGrayscaler.ConvertToGrayscale(image);
long stopTime = System.currentTimeMillis();
long elapsedTime = stopTime - startTime;
System.out.println("Sequential code executed in " + elapsedTime + " ms.");
// Multithreaded (parallel):
startTime = System.currentTimeMillis();
parallelGrayscaler.start();
parallelGrayscaler2.start();
parallelGrayscaler3.start();
parallelGrayscaler4.start();
// Main waits for threads to finish so that the program doesn't "end" (i.e. stop measuring time) before the threads finish
parallelGrayscaler.join();
parallelGrayscaler2.join();
parallelGrayscaler3.join();
parallelGrayscaler4.join();
stopTime = System.currentTimeMillis();
elapsedTime = stopTime - startTime;
System.out.println("Multithreaded (parallel) code executed in " + elapsedTime + " ms.");
}
}
Parallel.java:
// Let each of the 4 threads work on a different quarter of the image
public class Parallel extends Thread{//implements Runnable{
private String threadName;
private static BufferedImage myImage; // Calling it "my" image because each thread will have its own unique quarter of the image to work on
private static int width, height; // Image params
Parallel(String name, BufferedImage image){
threadName = name;
System.out.println("Creating "+ threadName);
myImage = image;
width = myImage.getWidth();
height = myImage.getHeight();
}
public void run(){
System.out.println("Running " + threadName);
// Pixel by pixel (for our quarter of the image)
for (int j = 0; j < height; j++){
for (int i = 0; i < width; i++){
// Traversing the image and converting the RGB values (doing the same thing as the sequential code but on a smaller scale)
Color c = new Color(myImage.getRGB(i,j));
int red = (int)(c.getRed() * 0.299);
int green = (int)(c.getGreen() * 0.587);
int blue = (int)(c.getBlue() * 0.114);
Color newColor = new Color(red + green + blue, red + green + blue, red + green + blue);
myImage.setRGB(i,j,newColor.getRGB()); // Write the new value for that pixel
}
}
File output = new File("src/pixelsmanipulation/"+threadName+"grayscale.jpg"); // Put it in a "lower level" folder so we can see it in the project view
try {
ImageIO.write(newImage, "jpg", output);
} catch (IOException ex) {
Logger.getLogger(Parallel.class.getName()).log(Level.SEVERE, null, ex);
}
System.out.println("Thread " + threadName + " exiting. ---");
}
}
我是一个Java多线程（以及使用BufferedImage）的初学者，只是好奇为什么它很慢。