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    <p>To answer a portion of your question;</p> <p>Java at start-up allocates a "heap" of memory, or a fixed size block (the -Xms parameter). It doesn't actually use all this memory right off the bat, but it tells the OS "I want this much memory". Then as you create objects and do work in the Java environment, it puts the created objects into this heap of pre-allocated memory. If that block of memory gets full then it will request a little more memory from the OS, up until the "max heap size" (the -Xmx parameter) is reached.</p> <p>Once that max size is reached, Java will no longer request more RAM from the OS, even if there is a lot free. If you try to create more objects, there is no heap space left, and you will get an OutOfMemory exception. Now if you are looking at Windows Task Manager or something like that, you'll see "java.exe" using X megs of memory. That sort-of corresponds to the amount of memory that it has requested for the heap, not really the amount of memory inside the heap thats used.</p> <p>In other words, I could write the application:</p> <pre><code>class myfirstjavaprog { public static void main(String args[]) { System.out.println("Hello World!"); } } </code></pre> <p>Which would basically take very little memory. But if I ran it with the cmd line:</p> <pre><code>java.exe myfirstjavaprog -Xms 1024M </code></pre> <p>then on startup java will immediately ask the OS for 1,024 MB of ram, and thats what will show in Windows Task Manager. In actuallity, that ram isnt being used, but java reserved it for later use.</p> <p>Conversely, if I had an app that tried to create a 10,000 byte large array:</p> <pre><code>class myfirstjavaprog { public static void main(String args[]) { byte[] myArray = new byte[10000]; } } </code></pre> <p>but ran it with the command line:</p> <pre><code>java.exe myfirstjavaprog -Xms 100 -Xmx 100 </code></pre> <p>Then Java could only alocate up to 100 bytes of memory. Since a 10,000 byte array won't fit into a 100 byte heap, that would throw an OutOfMemory exception, even though the OS has plenty of RAM.</p> <p>I hope that makes sense...</p> <hr> <p>Edit:</p> <p>Going back to "why Java uses so much memory"; why do you think its using a lot of memory? If you are looking at what the OS reports, then that isn't what its actually using, its only what its reserved for use. If you want to know what java has actually used, then you can do a heap dump and explore every object in the heap and see how much memory its using.</p> <p>To answer "why doesn't it just let the OS handle it?", well I guess that is just a fundamental Java question for those that designed it. The way I look at it; Java runs in the JVM, which is a virtual machine. If you create a VMWare instance or just about any other "virtualization" of a system, you usually have to specify how much memory that virtual system will/can consume. I consider the JVM to be similar. Also, this abstracted memory model lets the JVM's for different OSes all act in a similar way. So for example Linux and Windows have different RAM allocation models, but the JVM can abstract that away and follow the same memory usage for the different OSes.</p>
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    1. POJava: why does it uses a fixed amount of memory? or how does it manage the memory?
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    1. COYea sure I understand the implications of these parameters. :) But that was not really my question. My question basically was: Why not let the OS take care of this. It does basically already the same thing.
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    2. CO@Albert: Set -Xmx to the system max and the system will (more or less) take care of it by giving it a lot of space to page into that it will probably never use. They could have set it higher by default if that's the question. Nothing stopping you from doing it on your system though.
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    3. CO@j flemm: Yes, this is one thing. This workaround would resolve the limit in the JVM. But the question was not really about how to workaround this limit but about why it is there. That was one part of my question. And the other part is why Java uses so much memory.
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