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() 返回我的进程正在使用的总内存。那是对的吗？

freeMemory() 和 maxMemory() 怎么样？

Answer 1

名称和值令人困惑。如果您正在寻找可用内存总量，您必须自己计算该值。 这不是你从freeMemory();得到的。

请参阅以下指南：

总指定内存，这将等于配置的 -Xmx 值：

Runtime.getRuntime().maxMemory();

当前分配的空闲内存，是当前为新对象分配的空间准备好。 注意这不是总的可用可用内存：

Runtime.getRuntime().freeMemory();

总分配内存，是为 java 进程分配的总空间保留：

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;

一张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时间变化，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。

关于您的问题，maxMemory() 返回 -Xmx 值。

您可能想知道为什么会有 totalMemory() 和 ma​​xMemory()。答案是 JVM 延迟分配内存。假设您这样启动 Java 进程：

java -Xms64m -Xmx1024m Foo

您的进程以 64mb 内存开始，如果需要更多内存（最多 1024m），它将分配内存。 totalMemory() 对应于 Foo 的 JVM当前可用的内存量。如果 JVM 需要更多内存，它会延迟地将其 up 分配到最大内存。如果使用-Xms1024m -Xmx1024m 运行，则从totalMemory() 和maxMemory() 获得的值将相等。

另外，如果您想准确计算已用内存的数量，您可以使用以下计算：

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

Answer 3

为了更好地理解它，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 used、free、total 和 maximum em> jvm 中可用的内存。

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（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());
    }
}

Answer 5

Runtime#totalMemory - JVM 到目前为止分配的内存。这不一定是正在使用的或最大值。

Runtime#maxMemory - JVM 已配置使用的最大内存量。一旦你的进程达到这个数量，JVM 就不会分配更多，而是更频繁地 GC。

Runtime#freeMemory - 我不确定这是从最大值还是未使用的总数中测量的部分。我猜这是对未使用的总数部分的度量。

Answer 6

JVM 堆大小可以通过垃圾收集机制进行增长和收缩。 但是，它不能分配超过最大内存大小：Runtime.maxMemory。这就是最大内存的意思。总内存是指分配的堆大小。而空闲内存是指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) 时分配堆 20M。在这种情况下，总内存为 20M。可用内存为 20M 已用大小。如果需要更多堆，JVM 分配更多但不能超过 50M(mx)。最大情况下，总内存为50M，空闲大小为50M-used size。至于minumum size(mn)，如果heap用得不多，jvm可以把heap size缩小到10M。

这种机制是为了提高内存效率。如果小型 java 程序运行在固定大小的巨大堆内存上，那么这么多内存可能是浪费的。

Answer 7

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");