blocks|key|243615|text|您可以随时调用Runtime.getRuntime().freeMemory()。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|243616|问题的另一半，获取对象的成本，对我来说似乎更有问题。|243617|我认为一个更好的解决方案是弄清楚如何集群和扩展你的web服务，这样它们就可以优雅地接受150%25的额定负载，而不会拒绝新的连接。听起来，规模调整练习会给你带来比代码黑客更好的解决方案。|243618|entityMap^0|7|X|0|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|O|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|P|8|@]|D|@]|E|$]]|$1|J|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|K|$]]

You can always call <code>Runtime.getRuntime().freeMemory()</code>.

The other half of the problem, getting the cost of objects, seems more problematic to me.

I think a better solution would be figuring out how to cluster and scale your web services so they can gracefully accept 150% of rated load without denying new connections. Sounds like a sizing exercise would get you a better solution than a code hack.

blocks|key|460613|text|Runtime.getRuntime().freeMemory()是一种在运行时获取free+memory+for+JVM的方法。这是不是好的方法呢?这完全取决于你的应用程序。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|460614|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.oracle.com/javase/6/docs/api/java/lang/Runtime.html^0|0|X|16|J|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@$9|O|A|P|B|C]]|D|@$9|Q|A|R|1|S]]|E|$]]|$1|F|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|G|$H|$5|I|J|K|E|$L|M]]]]

<code>Runtime.getRuntime().freeMemory()</code> is a way to get <a href="http://docs.oracle.com/javase/6/docs/api/java/lang/Runtime.html" rel="nofollow">free memory for JVM</a> at that moment while runtime. Is it good way (or) not it depends completely on your application.

blocks|key|243691|text|long+freeMemory+=+Runtime.getRuntime().freeMemory();|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|243692|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>long freeMemory = Runtime.getRuntime().freeMemory();
</code></pre>

blocks|key|243722|text|除了另一个答案，我想指出这样做不一定是一个好主意，因为你的应用程序中可能有一个使用SoftReferences的缓存。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|243723|这样的缓存将在JVM达到其内存限制时立即释放内存。分配内存，即使没有足够的空闲内存，也会首先导致软引用释放内存，并使其可用于分配。|243724|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.oracle.com/javase/6/docs/api/java/lang/ref/SoftReference.html^0|15|E|0|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@$A|O|B|P|1|Q]]|C|$]]|$1|D|3|E|5|6|7|R|8|@]|9|@]|C|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|C|$]]]|G|$H|$5|I|J|K|C|$L|M]]]]

In addition to the other answer, I would like to note that doing that is not necessarily a good idea, since you might have a cache in your app that uses <a href="http://docs.oracle.com/javase/6/docs/api/java/lang/ref/SoftReference.html">SoftReferences</a>. 

Such a cache would release memory as soon as the JVM reaches its memory limits. Allocating memory, even if there's not enough free memory, would first cause memory to be released by the soft references, and make it available for the allocation.

blocks|key|243789|text|除了使用Runtime方法之外，还可以使用以下命令获取一些额外的内存信息|type|unstyled|depth|inlineStyleRanges|entityRanges|data|243790|MemoryMXBean+memBean+=+ManagementFactory.getMemoryMXBean();
MemoryUsage+heap+=+memBean.getHeapMemoryUsage();
MemoryUsage+nonheap+=+memBean.getNonHeapMemoryUsage();|code-block|syntax|javascript|243791|每个MemoryUsage都提供初始化、使用、提交和最大值。如果创建一个内存监视器线程来轮询内存并记录它，向您提供一段时间内内存使用的历史记录，这可能会很有用。有时，查看导致错误的一段时间内的内存使用情况会很有帮助。|offset|length|243792|如果你真的想把这一点发挥到极致，那就创建一个堆转储线程。监控一段时间内的内存使用情况，当它超过特定阈值时，请执行以下操作(这适用于JBoss+5.0+-您的里程可能会有所不同)：|243793|//+init+code
MBeanServer+server+=+ManagementFactory.getPlatformMBeanServer();
HotSpotDiagnosticMXBean+diagBean+=+ManagementFactory.newPlatformMXBeanProxy(server,+"com.sun.management:type=HotSpotDiagnostic",+HotSpotDiagnosticMXBean.class);+

//+loop+code
//+add+some+code+to+figure+if+we+have+passed+some+threshold,+then

File+heapFile+=+new+File(outputDir,+"heap-"+%2B+curThreshold+%2B+".hprof");
log.info("Dumping+heap+file+"+%2B+heapFile.getAbsolutePath());
diagBean.dumpHeap(heapFile.getAbsolutePath(),+true);|243794|稍后，您可以使用eclipse+memory+analyzer或类似的工具检查这些堆转储文件，以检查内存泄漏等。|243795|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/management/MemoryUsage.html|1|http://www.eclipse.org/mat/^0|0|0|2|B|0|0|0|0|G|F|1|0^^$0|@$1|2|3|4|5|6|7|10|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|11|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|12|8|@]|9|@$I|13|J|14|1|15]]|A|$]]|$1|K|3|L|5|6|7|16|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|17|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|18|8|@]|9|@$I|19|J|1A|1|1B]]|A|$]]|$1|Q|3|-4|5|6|7|1C|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|X]]|Y|$5|T|U|V|A|$W|Z]]]]

In addition to using the Runtime methods, you can get some additional memory info using

<pre><code>MemoryMXBean memBean = ManagementFactory.getMemoryMXBean();
MemoryUsage heap = memBean.getHeapMemoryUsage();
MemoryUsage nonheap = memBean.getNonHeapMemoryUsage();
</code></pre>

Each <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/management/MemoryUsage.html">MemoryUsage</a> provides init, used, committed and max values. This might be useful if create a memory monitor thread which polls memory and logs it, providing you with a history of memory usage over time. Sometimes it's helpful to see the memory usage over time leading up to the errors.

If you really want to take this to an extreme, create a heap dump thread. Monitor your memory usage over time and when it exceeds certain thresholds do the following (this works on JBoss 5.0 - your mileage may vary):

<pre><code>// init code
MBeanServer server = ManagementFactory.getPlatformMBeanServer();
HotSpotDiagnosticMXBean diagBean = ManagementFactory.newPlatformMXBeanProxy(server, "com.sun.management:type=HotSpotDiagnostic", HotSpotDiagnosticMXBean.class); 

// loop code
// add some code to figure if we have passed some threshold, then

File heapFile = new File(outputDir, "heap-" + curThreshold + ".hprof");
log.info("Dumping heap file " + heapFile.getAbsolutePath());
diagBean.dumpHeap(heapFile.getAbsolutePath(), true);
</code></pre>

Later you can review these heap dump files with the eclipse <a href="http://www.eclipse.org/mat/">memory analyzer</a> or similar tools to check for memory leaks etc.

blocks|key|460734|text|要获得操作系统范围内的可用内存，请使用以下maven依赖项添加OSHI：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|460735|<dependency>
++++<groupId>com.github.oshi</groupId>
++++<artifactId>oshi-core</artifactId>
++++<version>LATEST</version>
</dependency>|code-block|syntax|javascript|460736|然后在Java中，使用以下代码：|460737|SystemInfo+si+=+new+SystemInfo();
HardwareAbstractionLayer+hal+=+si.getHardware();
long+availableMemory+=+hal.getMemory().getAvailable();|460738|entityMap|0|LINK|mutability|MUTABLE|url|https://mvnrepository.com/artifact/com.github.dblock/oshi-core^0|V|4|V|4|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]]|D|@$9|Z|A|10|1|11]]|E|$]]|$1|F|3|G|5|H|7|12|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|13|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|14|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|15|8|@]|D|@]|E|$]]]|P|$Q|$5|R|S|T|E|$U|V]]]]

To get the OS-wide available memory, add <a href="https://mvnrepository.com/artifact/com.github.dblock/oshi-core" rel="noreferrer"><code>OSHI</code></a> by using the following maven dependency: 

<pre><code>&lt;dependency&gt;
 &lt;groupId&gt;com.github.oshi&lt;/groupId&gt;
 &lt;artifactId&gt;oshi-core&lt;/artifactId&gt;
 &lt;version&gt;LATEST&lt;/version&gt;
&lt;/dependency&gt;
</code></pre>

Then in Java, use the following code:

<pre><code>SystemInfo si = new SystemInfo();
HardwareAbstractionLayer hal = si.getHardware();
long availableMemory = hal.getMemory().getAvailable();
</code></pre>

Is there any good way to get the remaining memory available to the JVM at run time? The use case of this would be to have web services which fail gracefully when they are nearing their memory limits by refusing new connections with a nice error message "too many people using this, try again later", rather than dying abruptly with an OutOfMemory error.

Note this has nothing to do with calculating/estimating the cost of each object beforehand. In principle I could estimate how much memory my objects take and refuse new connections based on that estimate, but that seems kind of hacky/fragile.

Java get available memory

Java

有没有什么好方法可以让JVM在运行时使用剩余的内存？这样做的用例是，当web服务接近内存限制时，它们会通过拒绝新连接来优雅地失败，并显示一条漂亮的错误消息"too many using this，try later"，而不是突然死于OutOfMemory错误。注意:这与预先计算/估计每个对象的成本无关。原则上，我可以估...

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