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

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()返回我的进程正在使用的总内存。那是对的吗？

如何freeMemory()与maxMemory()？

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时间变化，具体取决于主机环境。注意，保存任何给定类型的对象所需的内存量可能取决于实现。

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，则将返回Long.MAX_VALUE值。

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。

参考您的问题，maxMemory()返回-Xmx值。

你可能会奇怪为什么有totalMemory（）和A maxMemory（） 。答案是JVM延迟分配内存。可以这样说来启动Java流程：

java -Xms64m -Xmx1024m Foo

您的进程以64mb的内存开始，如果需要更多内存（最大1024m），它将分配内存。 totalMemory()对应于JVM for Foo 当前可用的内存量。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。如果使用-Xms1024m -Xmx1024m，则您从totalMemory()和获得的价值maxMemory()将等于。

另外，如果要准确计算已用内存量，请执行以下计算：

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

名称和值令人困惑。如果您要寻找总的可用内存，则必须自行计算该值。这不是你从中得到的freeMemory();。

请参阅以下指南：

指定的总内存，这将等于配置的-Xmx值：

Runtime.getRuntime（）。maxMemory（）;

当前分配的可用内存是为新对象准备的当前分配空间。警告这不是可用的总可用内存：

Runtime.getRuntime（）。freeMemory（）;

总分配内存，是为Java进程保留的总分配空间：

Runtime.getRuntime（）。totalMemory（）;

已使用的内存，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;

图片可能有助于阐明：

为了更好地理解它，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");            
        }
}

所有其他答案的编纂版本（在撰写本文时）：

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());
    }
}

运行时#totalMemory到目前为止，JVM已分配的内存。这不一定是正在使用或最大的使用。

运行时#maxMemory已配置JVM使用的最大内存量。一旦您的进程达到此数量，JVM将不会分配更多资源，而是会更频繁地分配GC。

Runtime＃freeMemory-我不确定这是从最大值还是未使用的总量中测得的。我猜这是对未使用的总计部分的度量。

JVM堆大小可以通过Garbage-Collection机制进行增长和收缩。但是，它不能分配最大内存大小：Runtime.maxMemory。这就是最大内存的含义。总内存意味着分配的堆大小。可用内存意味着总内存中的可用大小。

示例）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。可用内存为20M用过的大小。如果需要更多堆，则JVM分配更多，但不能超过50M（mx）。在最大的情况下，总内存为50M，可用大小为50M使用的大小。至于最小大小（mn），如果不使用堆，jvm可以将堆大小缩小到10M。

这种机制是为了提高存储效率。如果小型Java程序在巨大的固定大小的堆内存上运行，那么太多的内存可能是浪费的。

您可以看到MB格式的结果，除以1024 x 1024，等于1 MB。

int dataSize = 1024 * 1024;

System.out.println("Used Memory   : " + (Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory())/dataSize + " MB");
System.out.println("Free Memory   : " + Runtime.getRuntime().freeMemory()/dataSize + " MB");
System.out.println("Total Memory  : " + Runtime.getRuntime().totalMemory()/dataSize + " MB");
System.out.println("Max Memory    : " + Runtime.getRuntime().maxMemory()/dataSize + " MB");