什么是Runtime.getRuntime().totalMemory()和freeMemory()？

Question

我一直想知道Runtime.getRuntime().totalMemory(),Runtime.getRuntime().freeMemory()和Runtime.getRuntime().maxMemory()的确切含义是什么 .

我的理解是,Runtime.getRuntime().totalMemory()返回我的进程使用的总内存.那是对的吗？

如何freeMemory()与maxMemory()？

Answer 1

名称和价值令人困惑.如果您正在寻找总可用内存,则必须自行计算此值.这不是你得到的freeMemory();.

请参阅以下指南:

总指定内存,这将等于配置的-Xmx值:

调用Runtime.getRuntime()maxMemory();

当前分配的空闲内存,是为新对象准备的当前分配空间.注意这不是可用的总内存:

调用Runtime.getRuntime()freeMemory();

已分配的内存总量,是为java进程保留的总分配空间:

调用Runtime.getRuntime()totalMemory();

已用内存,必须计算:

usedMemory = Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory();

总可用内存,必须计算:

freeMemory = Runtime.getRuntime().maxMemory() - usedMemory;

图片可能有助于澄清:

Answer 2

根据API

totalMemory()

返回Java虚拟机中的内存总量.此方法返回的值可能会随着时间的推移而变化,具体取决于主机环境.请注意,保存任何给定类型的对象所需的内存量可能与实现有关.

maxMemory()

返回Java虚拟机将尝试使用的最大内存量.如果没有固有限制,则返回值Long.MAX_VALUE.

freeMemory()

返回Java虚拟机中的可用内存量.调用gc方法可能会导致freeMemory返回的值增加.

在参考您的问题时,maxMemory()返回-Xmx值.

你可能会奇怪为什么有totalMemory()和A maxMemory() .答案是JVM懒惰地分配内存.让我们说你开始你的Java过程:

java -Xms64m -Xmx1024m Foo

你的进程以64mb的内存开始,如果它需要更多(最多1024m),它将分配内存. totalMemory()对应于JVM 当前可用于Foo 的内存量.如果JVM需要更多的内存,它会懒洋洋地分配它达到最大内存.如果你运行-Xms1024m -Xmx1024m,你获得的价值totalMemory()和maxMemory()将等于.

此外,如果要准确计算已用内存量,请执行以下计算:

final long usedMem = totalMemory() - freeMemory();

Answer 3

要更好地理解它,请运行以下程序(在jdk1.7.x中):

$ java -Xms1025k -Xmx1025k -XshowSettings:vm  MemoryTest

这将打印JVM选项和使用的,免费的,总的和最大的JVM可用内存.

public class MemoryTest {    
    public static void main(String args[]) {
                System.out.println("Used Memory   :  " + (Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory()) + " bytes");
                System.out.println("Free Memory   : " + Runtime.getRuntime().freeMemory() + " bytes");
                System.out.println("Total Memory  : " + Runtime.getRuntime().totalMemory() + " bytes");
                System.out.println("Max Memory    : " + Runtime.getRuntime().maxMemory() + " bytes");            
        }
}

Answer 4

所有其他答案的编纂版本(撰写本文时):

import java.io.*;

/**
 * This class is based on <a href="http://stackoverflow.com/users/2478930/cheneym">cheneym</a>'s
 * <a href="http://stackoverflow.com/a/18375641/253468">awesome interpretation</a>
 * of the Java {@link Runtime}'s memory query methods, which reflects intuitive thinking.
 * Also includes comments and observations from others on the same question, and my own experience.
 * <p>
 * <img src="https://i.stack.imgur.com/GjuwM.png" alt="Runtime's memory interpretation">
 * <p>
 * <b>JVM memory management crash course</b>:
 * Java virtual machine process' heap size is bounded by the maximum memory allowed.
 * The startup and maximum size can be configured by JVM arguments.
 * JVMs don't allocate the maximum memory on startup as the program running may never require that.
 * This is to be a good player and not waste system resources unnecessarily.
 * Instead they allocate some memory and then grow when new allocations require it.
 * The garbage collector will be run at times to clean up unused objects to prevent this growing.
 * Many parameters of this management such as when to grow/shrink or which GC to use
 * can be tuned via advanced configuration parameters on JVM startup.
 *
 * @see <a href="http://stackoverflow.com/a/42567450/253468">
 *     What are Runtime.getRuntime().totalMemory() and freeMemory()?</a>
 * @see <a href="http://www.oracle.com/technetwork/java/javase/memorymanagement-whitepaper-150215.pdf">
 *     Memory Management in the Sun Java HotSpot™ Virtual Machine</a>
 * @see <a href="http://docs.oracle.com/javase/8/docs/technotes/tools/windows/java.html">
 *     Full VM options reference for Windows</a>
 * @see <a href="http://docs.oracle.com/javase/8/docs/technotes/tools/unix/java.html">
 *     Full VM options reference for Linux, Mac OS X and Solaris</a>
 * @see <a href="http://www.oracle.com/technetwork/articles/java/vmoptions-jsp-140102.html">
 *     Java HotSpot VM Options quick reference</a>
 */
public class SystemMemory {

    // can be white-box mocked for testing
    private final Runtime runtime = Runtime.getRuntime();

    /**
     * <b>Total allocated memory</b>: space currently reserved for the JVM heap within the process.
     * <p>
     * <i>Caution</i>: this is not the total memory, the JVM may grow the heap for new allocations.
     */
    public long getAllocatedTotal() {
        return runtime.totalMemory();
    }

    /**
     * <b>Current allocated free memory</b>: space immediately ready for new objects.
     * <p>
     * <i>Caution</i>: this is not the total free available memory,
     * the JVM may grow the heap for new allocations.
     */
    public long getAllocatedFree() {
        return runtime.freeMemory();
    }

    /**
     * <b>Used memory</b>:
     * Java heap currently used by instantiated objects. 
     * <p>
     * <i>Caution</i>: May include no longer referenced objects, soft references, etc.
     * that will be swept away by the next garbage collection.
     */
    public long getUsed() {
        return getAllocatedTotal() - getAllocatedFree();
    }

    /**
     * <b>Maximum allocation</b>: the process' allocated memory will not grow any further.
     * <p>
     * <i>Caution</i>: This may change over time, do not cache it!
     * There are some JVMs / garbage collectors that can shrink the allocated process memory.
     * <p>
     * <i>Caution</i>: If this is true, the JVM will likely run GC more often.
     */
    public boolean isAtMaximumAllocation() {
        return getAllocatedTotal() == getTotal();
        // = return getUnallocated() == 0;
    }

    /**
     * <b>Unallocated memory</b>: amount of space the process' heap can grow.
     */
    public long getUnallocated() {
        return getTotal() - getAllocatedTotal();
    }

    /**
     * <b>Total designated memory</b>: this will equal the configured {@code -Xmx} value.
     * <p>
     * <i>Caution</i>: You can never allocate more memory than this, unless you use native code.
     */
    public long getTotal() {
        return runtime.maxMemory();
    }

    /**
     * <b>Total free memory</b>: memory available for new Objects,
     * even at the cost of growing the allocated memory of the process.
     */
    public long getFree() {
        return getTotal() - getUsed();
        // = return getAllocatedFree() + getUnallocated();
    }

    /**
     * <b>Unbounded memory</b>: there is no inherent limit on free memory.
     */
    public boolean isBounded() {
        return getTotal() != Long.MAX_VALUE;
    }

    /**
     * Dump of the current state for debugging or understanding the memory divisions.
     * <p>
     * <i>Caution</i>: Numbers may not match up exactly as state may change during the call.
     */
    public String getCurrentStats() {
        StringWriter backing = new StringWriter();
        PrintWriter out = new PrintWriter(backing, false);
        out.printf("Total: allocated %,d (%.1f%%) out of possible %,d; %s, %s %,d%n",
                getAllocatedTotal(),
                (float)getAllocatedTotal() / (float)getTotal() * 100,
                getTotal(),
                isBounded()? "bounded" : "unbounded",
                isAtMaximumAllocation()? "maxed out" : "can grow",
                getUnallocated()
        );
        out.printf("Used: %,d; %.1f%% of total (%,d); %.1f%% of allocated (%,d)%n",
                getUsed(),
                (float)getUsed() / (float)getTotal() * 100,
                getTotal(),
                (float)getUsed() / (float)getAllocatedTotal() * 100,
                getAllocatedTotal()
        );
        out.printf("Free: %,d (%.1f%%) out of %,d total; %,d (%.1f%%) out of %,d allocated%n",
                getFree(),
                (float)getFree() / (float)getTotal() * 100,
                getTotal(),
                getAllocatedFree(),
                (float)getAllocatedFree() / (float)getAllocatedTotal() * 100,
                getAllocatedTotal()
        );
        out.flush();
        return backing.toString();
    }

    public static void main(String... args) {
        SystemMemory memory = new SystemMemory();
        System.out.println(memory.getCurrentStats());
    }
}

Answer 5

Runtime#totalMemory - 到目前为止JVM分配的内存.这不一定是使用中的或最大的.

Runtime#maxMemory - JVM配置使用的最大内存量.一旦您的流程达到此数量,JVM将不会更频繁地分配更多,而是GC.

Runtime#freeMemory - 我不确定这是从最大值还是未使用总量的部分来衡量的.我猜测它是未使用的总量的一部分.

Answer 6

垃圾收集机制可以使JVM堆大小可扩展和收缩.但是,它无法分配超过最大内存大小:Runtime.maxMemory.这是最大记忆的意义.总内存意味着分配的堆大小.空闲内存意味着总内存中可用的大小.

例子)java -Xms20M -Xmn10M -Xmx50M ~~~.这意味着jvm应该在start(ms)上分配堆20M.在这种情况下,总内存为20M.空闲内存是20M使用的大小.如果需要更多堆,JVM分配更多但不能超过50M(mx).在最大值的情况下,总内存为50M,自由大小为50M使用的大小.至于最小大小(mn),如果堆使用不多,jvm可以将堆大小缩小到10M.

这种机制是为了提高记忆效率.如果小型java程序在巨大的固定大小堆内存上运行,那么大量内存可能会浪费.