为什么这个 Java 程序占用这么多内存？

Question

我有一小段代码每五分钟对我的桌面进行一次屏幕截图。 但是，我对它占用的内存量感到有些困惑-通常它会爬到 200mb 的 RAM，我敢肯定这是过多的...谁能告诉我 a) 减少内存占用的明智方法或b) 为什么它会上升？

/**
 * Code modified from code given in http://whileonefork.blogspot.co.uk/2011/02/java-multi-monitor-screenshots.html following a SE question at  
 * http://*.com/questions/10042086/screen-capture-in-java-not-capturing-whole-screen and then modified by a code review at http://codereview.stackexchange.com/questions/10783/java-screengrab
 */
package com.tmc.personal;

import java.awt.AWTException;
import java.awt.GraphicsDevice;
import java.awt.GraphicsEnvironment;
import java.awt.Rectangle;
import java.awt.Robot;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.TimeZone;
import java.util.concurrent.TimeUnit;

import javax.imageio.ImageIO;

class ScreenCapture {

    static int minsBetweenScreenshots = 5;

    public static void main(String args[]) {
        int indexOfPicture = 1000;// should be only used for naming file...
        while (true) {
            takeScreenshot("ScreenCapture" + indexOfPicture++);
            try {
                TimeUnit.MINUTES.sleep(minsBetweenScreenshots);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    //from http://www.coderanch.com/t/409980/java/java/append-file-timestamp
    private  final static String getDateTime()
    {
        DateFormat df = new SimpleDateFormat("yyyy-MM-dd_hh:mm:ss");
        df.setTimeZone(TimeZone.getTimeZone("PST"));
        return df.format(new Date());
    }

    public static void takeScreenshot(String filename) {
        Rectangle allScreenBounds = getAllScreenBounds();
        Robot robot;
        try {
            robot = new Robot();
            BufferedImage screenShot = robot.createScreenCapture(allScreenBounds);
            ImageIO.write(screenShot, "jpg", new File(filename + getDateTime()+ ".jpg"));
        } catch (AWTException e) {
            System.err.println("Something went wrong starting the robot");
            e.printStackTrace();
        } catch (IOException e) {
            System.err.println("Something went wrong writing files");
            e.printStackTrace();
        }
    }

    /**
     * Okay so all we have to do here is find the screen with the lowest x, the
     * screen with the lowest y, the screen with the higtest value of X+ width
     * and the screen with the highest value of Y+height
     * 
     * @return A rectangle that covers the all screens that might be nearby...
     */
    private static Rectangle getAllScreenBounds() {
        Rectangle allScreenBounds = new Rectangle();
        GraphicsEnvironment ge = GraphicsEnvironment.getLocalGraphicsEnvironment();
        GraphicsDevice[] screens = ge.getScreenDevices();

        int farx = 0;
        int fary = 0;
        for (GraphicsDevice screen : screens) {
            Rectangle screenBounds = screen.getDefaultConfiguration().getBounds();
            // finding the one corner
            if (allScreenBounds.x > screenBounds.x) {
                allScreenBounds.x = screenBounds.x;
            }
            if (allScreenBounds.y > screenBounds.y) {
                allScreenBounds.y = screenBounds.y;
            }
            // finding the other corner
            if (farx < (screenBounds.x + screenBounds.width)) {
                farx = screenBounds.x + screenBounds.width;
            }
            if (fary < (screenBounds.y + screenBounds.height)) {
                fary = screenBounds.y + screenBounds.height;
            }
            allScreenBounds.width = farx - allScreenBounds.x;
            allScreenBounds.height = fary - allScreenBounds.y;
        }
        return allScreenBounds;
    }
}

Answer 1

其他答案是正确的，Java 将使用尽可能多的内存，此时它将进行垃圾收集。要解决此问题，您可以在 JVM 设置中指定较小的最大堆大小。您可以使用 -Xmx 设置来执行此操作。例如，如果你认为你只需要 32MB，那么运行它：

java -Xmx32M [your main class or jar here]

你的程序堆（非堆栈内存）永远不会超过 32MB，但如果它一次需要超过 32MB，它会崩溃（这就是你需要分析的地方）。不过，我没有在您的程序中看到任何明显的泄漏（假设 ImageIO 不需要任何清理），所以我认为您会没事的。

Answer 2

JVM 垃圾收集器最终会清除你的内存堆。用于手动清除该堆调用 Runtime.getRuntime().gc();，但我不建议每 5 分钟执行一次。

Answer 3

对于现代计算机，200MB 的内存并不算多。如果您要创建和丢弃大量对象，JVM 会让堆增长一段时间，这样您的程序就不会陷入垃圾收集的困境。让您的程序运行几个小时，然后检查是否有问题。