Most HotSpot JVM in the last year have had it on by default. This option allows references to be 32-bit in a 64-bit JVM and access close to 32 GB of heap. (more than 32-bit pointers can) (You can have near unlimited off heap memory as well).  This can save a significant amount of memory and potentially improve performance.

If you want to use this option I suggest you update to a version which has it on by default as there may have been a good reason, such as bugs, why it wasn&#39;t enabled previously. Try Java 6 update 23 or Java 7 update 5.

In short, don&#39;t turn it on, use a version which has it on by default.

---

Update:

In Java 8 you have the option to set the `-XX:ObjectAlignmentInBytes=` and in fact if you heap size to 64 GB it will use `-XX:ObjectAlignmentInBytes=16` and still use 32-bit references.

Consider the following scenario:

- Shared Library libA.so ,with no dependencies.
- Shared Library libB.so, with libA.so as its dependency.

I want to compile a binary file that links with the libB.
Should I link the binary with libB only or with libA either?

Is there any way to link only with the direct dependencies, letting the resolution of unresolved symbols from the dependencies for runtime?

I&#39;m worried about the fact that the library libB implementation may change in the future, introducing other dependencies (libC, libD, libE for instance). Am I going to have problems with that?

In other words:

- libA files: a.cpp a.h
- libB files: b.cpp b.h
- main program files: main.cpp

Of course, b.cpp includes a.h and main.cpp includes b.h.

Compilation commands:

    g++ -fPIC a.cpp -c
    g++ -shared -o libA.so a.o
    
    g++ -fPIC b.cpp -c -I.
    g++ -shared -o libB.so b.o -L. -lA

Which of the bellow options should I use?

    g++ main.cpp -o main -I. -L. -lB

or

    g++ main.cpp -o main -I. -L. -lB -lA

I couldn&#39;t use the first option. The linker complains about the unresolved symbols from the library libA. But it sound a little strange to me.

Thanks very much.


**-- Updated comments:**


When I link the binary, the linker will try to resolve all symbols from the main and the libB. However, libB has undefined symbols from the libA. That&#39;s why the linker complains about that.

That&#39;s why I need to link with the libA too.
However I found a way to ignore unresolved symbols from shared libraries.
Looks like I should use the following command line to do that:

    g++ main.cpp -o main -I. -L. -lB -Wl,-unresolved-symbols=ignore-in-shared-libs

Looks like it is still possible to use the `-rpath` option.
However I need to understand it a little better.

Does anyone knows any possible pitfalls when using the `-Wl,-unresolved-symbols=ignore-in-shared-libs` option?


**-- Updated comments 2:**


`-rpath` should not be used for this purpose. It is useful to force a library to be found in a given directory. The `-unresolved-symbol` approach looks much better.


Thanks again.

Linking with dynamic library with dependencies

Has Microsoft created a class full of constants for the standard HTTP header names or will I have to write my own?

Are there any constants for the default HTTP headers?

What does the HotSpot JVM flag `-XX:+UseCompressedOops` do and when should I use it? What sort of performance and memory-usage differences will I see when using it on a 64-bit Java instance (vs. not using it)? 

What does the UseCompressedOops JVM flag do and when should I use it?

<p>What does the HotSpot JVM flag <code>-XX:+UseCompressedOops</code> do and when should I use it? What sort of performance and memory-usage differences will I see when using it on a 64-bit Java instance (vs. not using it)?</p>


The icon that IntelliJ (v11.1) usually shows next to Java classes is a blue circle with a C. But for one of my classes this icon has a small &quot;x&quot; in the top left corner. This particular class is in a package with another class, which has just the usual C in a blue circle.

In case it matters, I&#39;m using a Mac.

The class in question is **AlertsHandler**.


![enter image description here][1]


  [1]: http://i.stack.imgur.com/RsJEH.png

Why is Intellij showing a cross (x) over the class icon for one of my Java classes?

I wanted code to convert all the characters in strings to uppercase or lowercase in Java.

I found a method that goes something like this:

    public static String changelowertoupper()
    {
             String str = &quot;CyBeRdRaGoN&quot;;
             str=str.toLowerCase(Locale.ENGLISH);
             return str;
    }
 
Now I&#39;ve read that using certain `Locale`s, like Turkish, &quot;returns ***i (without dot)*** instead of ***i (with dot)***.&quot;

Is it safe to use `Locale`s like UK, US, ENGLISH, etc.? Are there any big differences between them when applied to strings?

Which is the most preferred `Locale` for `String`s?

Using Locales with Java&#39;s toLowerCase() and toUpperCase()

I have this common issue, as it appears to be. My table view wont refresh my items after I reset them. I have checked the data and it&#39;s the new one. 

**I tried multiple solution from internet but no success.** 

Can&#39;t reset all the columns because it adds one empty one extra (dont know why) and the resize just breakes.

My table is not **editable**. The new data is changed.

The data is refreshed if I **change the ordering** of the items and the rows change (:|).

I&#39;m just left without ideas.

At the moment the refresh code is pretty simple.


    ObservableList&lt;User&gt; data = FXCollections.observableArrayList(User.getResellers());
    reseller_table.setItems(data);

Again the new data is correct. When I make an selection to the tableView it returns the new correct Item.

JavaFX 2.1 TableView refresh items

&gt; **Possible Duplicate:**  
&gt; [Why use getters and setters?](https://stackoverflow.com/questions/1568091/why-use-getters-and-setters)  

&lt;!-- End of automatically inserted text --&gt;

Is there any advantage to making methods to access private variables in your class instead of making the variable public?

For example is the second case better than the first?

    //Case 1
    public class Shoe{
        public int size;
    }
    
    //Case 2
    public class Shoe{
        private int size;
        public int getSize(){
            return size;
        }
    
        public void setSize(int sz){
            size = sz;
        }
    
    }

Advantage of set and get methods vs public variable

When I started to look for the benefits of polymorphism, I found with [this](https://stackoverflow.com/questions/11064409/why-to-use-polymorphism) question here. But here I was unable to find my answer. Let me tell what I want to find. Here I have some classes:

    class CoolingMachines{
    	public void startMachine(){
    		//No implementationion
    	}
    	public void stopMachine(){
    		//No implementationion
    	}
    }
    
    class Refrigerator extends CoolingMachines{
    	public void startMachine(){
    		System.out.println(&quot;Refrigerator Starts&quot;);
    	}
    	public void stopMachine(){
    		System.out.println(&quot;Refrigerator Stop&quot;);
    	}
    	public void trip(){
    		System.out.println(&quot;Refrigerator Trip&quot;);
    	}
    }
    
    class AirConditioner extends CoolingMachines{
    	public void startMachine(){
    		System.out.println(&quot;AC Starts&quot;);
    	}
    	public void stopMachine(){
    		System.out.println(&quot;AC Stop&quot;);
    	}
    }

    public class PolymorphismDemo {
    	CoolingMachines cm = new Refrigerator();
    	Refrigerator rf = new Refrigerator();
    }

Now here I created two objects in the Demo class and are references of `Refrigerator`. I have completely understood that from the `rf` object I am able to call the `trip()` method of `Refrigerator`, but that method will be hidden for the `cm` object. Now my question is why should I use polymorphism or why should I use

    CoolingMachines cm = new Refrigerator();

when I am OK with

    Refrigerator rf = new Refrigerator();

Is polymorphic object&#39;s efficiency is good or light in weight? What is the basic purpose and  difference between both of these objects? Is there any difference between `cm.start();` and `rf.start()`?

Benefit of Polymorphism

I have a routine that runs different queries against an SQLite database many times per second. After a while I would get the error 

`&quot;android.database.CursorWindowAllocationException: - Cursor window allocation of 2048 kb failed. # Open Cursors = &quot;` appear in LogCat.

 I had the app log memory usage, and indeed when usage reaches a certain limit the I get this error, implying it runs out. My intuition tells me that the database engine is creating a NEW buffer (CursorWindow) every time I run a query, and even though .close() the cursors, neither the garbage collector nor `SQLiteDatabase.releaseMemory()` are quick enough at freeing memory. I think the solution may lie in &quot;forcing&quot; the database to always write into the same buffer, and not create new ones, but I have been unable to find a way to do this. I have tried instantiating my own CursorWindow, and tried setting SQLiteCursor to it, but to no avail.

&#191;Any ideas?

EDIT: re example code request from @GrahamBorland:

    public static CursorWindow cursorWindow = new CursorWindow(&quot;cursorWindow&quot;); 
    public static SQLiteCursor sqlCursor;
    public static void getItemsVisibleArea(GeoPoint mapCenter, int latSpan, int lonSpan) {
    query = &quot;SELECT * FROM Items&quot;; //would be more complex in real code
    sqlCursor = (SQLiteCursor)db.rawQuery(query, null);
    sqlCursor.setWindow(cursorWindow);
    }

Ideally I would like to be able to `.setWindow()` before giving a new query, and have the data put into the same `CursorWindow` everytime I get new data. 

SQLite Android Database Cursor window allocation of 2048 kb failed

I don&#39;t understand the difference between assignment constructor and copy constructor in C++. It is like this:

&lt;!-- language: cpp --&gt;

    class A {
    public:
        A() {
            cout &lt;&lt; &quot;A::A()&quot; &lt;&lt; endl;
        }
    };

    // The copy constructor
    A a = b;

    // The assignment constructor
    A c;
    c = a;

    // Is it right?


I want to know how to allocate memory of the assignment constructor and copy constructor?


What&#39;s the difference between assignment operator and copy constructor?

I have a large .xlsx file (141 MB, containing 293413 lines with 62 columns each)  I need to perform some operations within.

I am having problems with loading this file (`OutOfMemoryError`), as POI has a large memory footprint on XSSF (xlsx) workbooks.

[This SO question][1] is similar, and the solution presented is to increase the VM&#39;s allocated/maximum memory.

It seems to work for that kind of file-size (9MB), but for me, it just simply doesn&#39;t work even if a allocate all available system memory. (Well, it&#39;s no surprise considering the file is over 15 times larger)

I&#39;d like to know if there is any way to load the workbook in a way it won&#39;t consume all the memory, and yet, without doing the processing based (going into) the XSSF&#39;s underlying XML. (In other words, maintaining a puritan POI solution)

If there isn&#39;t tough, you are welcome to say it (&quot;There isn&#39;t.&quot;) and point me the ways to a &quot;XML&quot; solution.

  [1]: https://stackoverflow.com/questions/6069847/java-lang-outofmemoryerror-java-heap-space-while-reading-excel-with-apache-poi

How to load a large xlsx file with Apache POI?

Since my question is getting longer and longer, I decide to re-write the whole question to make it better and shorter.

I run my website on dedicated server with 8GB memory. I am fully aware that I need to raise the memory limit on php.ini setting. I have set it from 128M to 256M and to -1. Still the problem is persistence.

&gt; Fatal error: Out of memory (allocated 786432) (tried to allocate 24576
&gt; bytes) in D:\www\football\views\main.php on line 81

The out of memory does not make sense because it said only 786432 bytes is allocated and it needed 24576 bytes more.

786432 bytes is only 768 kilobytes and is fairly small.

&lt;h2&gt;Hints&lt;/h2&gt;

 - The error occurs on a very random line. It does not always error on line number 81.
 - At peak time, Apache only takes around 500mb of memory. I still have 6GB to spare.
 - There is no infinite loop.
 - The script takes 1,042,424 bytes. Getting this number from `echo memory_get_peak_usage();`
 - The resultset from MySQL is small (at most 12 of rows, purely text, no blob data)
 - (**Important**) If I restart Apache once every two days, the error is gone. It usually happens when Apache is running more than 2 days.
 - I have included the profiling the script and you can get it [here][1].
 - This dedicated server is purely used to run only one website. This website is a high traffic website with average of 1,000 visitors every minute. At peak time, there will be 1,700 to 2,000 visitors accessing at the same time.

&lt;h2&gt;Server Spec&lt;/h2&gt;

&gt; OS: Windows 2008 R2 64-Bit   
&gt; CPU: Intel Core i5 - 4 cores   
&gt; RAM: 8 GB  
&gt; Apache 2.2    
&gt; PHP 5.3.1   
&gt; Storage: 2 x 1 TB hard drives   
&gt; Bandwidth: 10 TB per month

&lt;h2&gt;Solution&lt;/h2&gt;
I have finally tuned up and fixed the problem and I would like to share it here what I have done to improve:

 1. `favicon.ico` was missing which mess up with my route engine. Although my route engine is very small, but by including `favicon.ico`, it helps reduce memory usage by not running my route engine. Most of part of my website has it and I forgot to put it for this new section.
 2. Limit `MaxRequestPerChild` helps. In my other dedicated server, I have my `MaxRequestPerChild` limited. For this server, I set it to 0. I always thought that each script is isolated. Lets say if my script takes 800kb to run. Upon its completion, Apache or PHP should free 800kb memory. It seem like it doesn&#39;t work this way. Limited `MaxRequestPerChild` does help to prevent memory leak by creating new process after limited `MaxRequestPerChild` and the old process is dying. This is my new setting.

        ThreadsPerChild      1500
        MaxRequestsPerChild  10000 
    
 3. `ob_flush();` does reduce slightly more memory. It does not help much but every bit of optimization helps.
 4. I have used `xdebug` which I have never used before as suggested by people who attempt to answer this question. I have to say it is great tool and I have optimized a few stuffs to make it run slightly faster.
 5. I have disable a few unnecessary Apache module. I am trying to disable it one by one and leave it a few days test to ensure it works perfectly before I disable another one. I do have all unnecessary PHP extension disable now.
 6. Most of my script in this server used traditional way (no template, no database layer, pure PHP, HTML, and legacy mysql_* function). To be honest, it runs very fast and used extremely small memory. However, maintenance the script is not very easy as the website is getting longer. I have tried to convert some parts of the website into proper framework (my own tiny framework). The reason that I used my own framework because it is tiny (3kb for the whole framework and include only what I need).
 7. **Switching to IIS7.5 solving this problem completely**.
 
  [1]: http://pastebin.com/1WRWCZRs

Fatal error: Out of memory, but I do have plenty of memory (PHP)

I would like some information on how to correctly think about C++11 closures and `std::function` in terms of how they are implemented and how memory is handled.

Although I don&#39;t believe in premature optimisation, I do have a habit of carefully considering the performance impact of my choices while writing new code.  I also do a fair amount of real-time programming, e.g. on microcontrollers and for audio systems, where non-deterministic memory allocation/deallocation pauses are to be avoided.

Therefore I&#39;d like to develop a better understanding of when to use or not use C++ lambdas.

My current understanding is that a lambda with no captured closure is exactly like a C callback.  However, when the environment is captured either by value or by reference, an anonymous object is created on the stack.  When a value-closure must be returned from a function, one wraps it in `std::function`.  What happens to the closure memory in this case?  Is it copied from the stack to the heap?  Is it freed whenever the `std::function` is freed, i.e., is it reference-counted like a `std::shared_ptr`?

I imagine that in a real-time system I could set up a chain of lambda functions, passing B as a continuation argument to A, so that a processing pipeline `A-&gt;B` is created.  In this case, the A and B closures would be allocated once.  Although I&#39;m not sure whether these would be allocated on the stack or the heap.  However in general this seems safe to use in a real-time system.  On the other hand if B constructs some lambda function C, which it returns, then the memory for C would be allocated and deallocated repeatedly, which would not be acceptable for real-time usage.

In pseudo-code, a DSP loop, which I think is going to be real-time safe.  I want to perform processing block A and then B, where A calls its argument.  Both these functions return `std::function` objects, so `f` will be a `std::function` object, where its environment is stored on the heap:

    auto f = A(B);  // A returns a function which calls B
                    // Memory for the function returned by A is on the heap?
                    // Note that A and B may maintain a state
                    // via mutable value-closure!
    for (t=0; t&lt;1000; t++) {
        y = f(t)
    }

And one which I think might be bad to use in real-time code:

    for (t=0; t&lt;1000; t++) {
        y = A(B)(t);
    }

And one where I think stack memory is likely used for the closure:

    freq = 220;
    A = 2;
    for (t=0; t&lt;1000; t++) {
        y = [=](int t){ return sin(t*freq)*A; }
    }

In the latter case the closure is constructed at each iteration of the loop, but unlike the previous example it is cheap because it is just like a function call, no heap allocations are made.  Moreover, I wonder if a compiler could &quot;lift&quot; the closure and make inlining optimisations.

Is this correct?  Thank you.

C++11 lambda implementation and memory model

I was trying to find out the difference between Apache Tomcat variables - `CATALINA_OPTS` and `JAVA_OPTS` in [SO][1] and surprised to see that there is no question/answer posted here yet.  So I thought of sharing it here (with answer) after finding out the difference.  Check the answer/difference below.

**NOTE:** At the time of this posting, we&#39;re running Apache Tomcat v6.0.10 with JDK 6u32 on CentOS5 64-bit arch.

  [1]: http://stackoverflow.com &quot;StackOverflow&quot;

CATALINA_OPTS vs JAVA_OPTS - What is the difference?

In a Java thread dump, you can see locks mentioned within stack traces.  
There seems to be three kinds of information:

1:

    - locked &lt;0x00002aab329f7fa0&gt; (a java.io.BufferedInputStream)

2:

    - waiting to lock &lt;0x00002aaaf4ff6fa0&gt; (a org.alfresco.repo.lock.LockServiceImpl)

3:

    - parking to wait for  &lt;0x00002aaafbf70bb8&gt; (a java.util.concurrent.SynchronousQueue$TransferStack)

* 1: The thread has obtained a lock on object 0x00002aab329f7fa0.
* 2&amp;3: Seem to say that the thread is waiting for the lock on said object to become available...  
but what is the difference 2 and 3?

Java thread dump: Difference between &quot;waiting to lock&quot; and &quot;parking to wait for&quot;?

I have this VM with tomcat, java, and grails in it. I&#39;ve been getting permgen errors so I looked around and found the solution:

    set JAVA_OPTS=&quot;-Xms256m -Xmx1024m -XX:PermSize=512m -XX:MaxPermSize=512m&quot;

I use SSH to access the vm and type the arguments above. I suppose that would fix the problem. Thing is, I wanted to make sure that I did it correctly. So I searched again on how I could check the current permSize and this is the solution I got:

    jinfo -flag MaxPermSize 6444

6444 is the pid, and as a response, I got this.

    -XX:MaxPermSize=85983232

Question: Is the value of the maxPermSize in bytes? because, if it is, then that would mean that the java_opts command didn&#39;t work. I am expecting to get 512m but 85983232 bytes = 82 mb.. Or am I seeing it wrong..? Can anybody enlighten me on this? D:

How do I properly set the permgen size?

What is the difference between **JVM**, **JDK**, **JRE** &amp; **OpenJDK**?

I was programming in Java and I encountered these phrases, what are the differences among them?

What is the difference between JVM, JDK, JRE &amp; OpenJDK?

What is the difference between running a Java application with&lt;br&gt;&lt;code&gt;java -cp CLASSPATH&lt;/code&gt; and &lt;code&gt;java -jar JAR_FILE_PATH&lt;/code&gt;? Is one of them preferred to the other for running a Java application? I mean which one of these ways is more expensive for JVM (according to their machine resources usage)?

Which one will cause JVM to spawn more threads while trying to run the application?

Differences between &quot;java -cp&quot; and &quot;java -jar&quot;?

What exactly is HotSpot and how does it relate to JVM and OpenJDK? Is it a library? What exactly does it do?

Also, what is the difference between OpenJDK and HotSpot?

Difference between JVM and HotSpot?

I upgraded to the latest **Java 7u40** on **MacOS X** and started getting the following message on the console when launching my application using Eclipse. The app works fine but I would like to find out the cause of the problem and hopefully a fix for it. 
  

    objc[10012]: Class JavaLaunchHelper is implemented in both /Library/Java/JavaVirtualMachines/jdk1.7.0_40.jdk/Contents/Home/bin/java and /Library/Java/JavaVirtualMachines/jdk1.7.0_40.jdk/Contents/Home/jre/lib/libinstrument.dylib. One of the two will be used. Which one is undefined.

Does anyone know why this message is printed and how to fix it?

Class JavaLaunchHelper is implemented in both ... libinstrument.dylib. One of the two will be used. Which one is undefined

Other than the more &#39;advanced&#39; GUI from Java mission control, how are they different?

At first glance they seem to offer very similar functionality (Interpreting JMX data and Memory/CPU profiling).

However, as they are both shipped with the JDK (I&#39;m using JDK 1.7.0_51 SE) I&#39;m assuming there are significant differences, otherwise they would be combined into a single solution. Especially as this increases the size of the JDK significantly.

Is Java Mission Control ultimately going to replace JVisualVM in the future?

What are the differences between JVisualVM and Java Mission Control?

I had to run `jmap` in order to take heap dump of my process. but `jvm` returned:

    Unable to open socket file: target process not responding or HotSpot VM not loaded
    The -F option can be used when the target process is not responding

So I used the `-F`:

    ./jmap -F -dump:format=b,file=heap.bin 10330
    Attaching to process ID 10331, please wait...
    Debugger attached successfully.
    Server compiler detected.
    JVM version is 24.51-b03
    Dumping heap to heap.bin ...

1. Using `-F`  is allright  for taking heap dump?
2. I am waiting 20 minutes and not finished yet. Any ideas why?

Running jmap getting Unable to open socket file

I was benchmarking some code, and I could not get it to run as fast as with [`java.math.BigInteger`][1], even when using the exact same algorithm.
So I copied [`java.math.BigInteger`][1] source into my own package and tried this:

    //import java.math.BigInteger;

    public class MultiplyTest {
    	public static void main(String[] args) {
	    	Random r = new Random(1);
     		long tm = 0, count = 0,result=0;
    		for (int i = 0; i &lt; 400000; i++) {
    			int s1 = 400, s2 = 400;
    			BigInteger a = new BigInteger(s1 * 8, r), b = new BigInteger(s2 * 8, r);
    			long tm1 = System.nanoTime();
    			BigInteger c = a.multiply(b);
    			if (i &gt; 100000) {
    				tm += System.nanoTime() - tm1;
    				count++;
    			}
    			result+=c.bitLength();
    		}
    		System.out.println((tm / count) + &quot;nsec/mul&quot;);
    		System.out.println(result); 
    	}
    }

When I run this (jdk 1.8.0_144-b01 on MacOS) it outputs:

    12089nsec/mul
    2559044166

When I run it with the import line uncommented:

    4098nsec/mul
    2559044166

It&#39;s almost three times as fast when using the JDK version of BigInteger versus my version, even if it&#39;s using the exact same code.

I&#39;ve examined the bytecode with javap, and compared compiler output when running with options:

    -Xbatch -XX:-TieredCompilation -XX:+PrintCompilation -XX:+UnlockDiagnosticVMOptions 
    -XX:+PrintInlining -XX:CICompilerCount=1

and both versions seem to generate the same code.
So is hotspot using some precomputed optimisations that I can&#39;t use in my code? I always understood that they don&#39;t.
What explains this difference?


  [1]: https://docs.oracle.com/javase/7/docs/api/java/math/BigInteger.html

Content Type	Original Author	Original Content on Stackoverflow
Question	noahlz	View Question on Stackoverflow
Solution 1 - Java	Peter Lawrey	View Answer on Stackoverflow

What does the UseCompressedOops JVM flag do and when should I use it?

Java Problem Overview

Java Solutions

Solution 1 - Java

Are there any constants for the default HTTP headers?

Linking with dynamic library with dependencies

Attributions