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Determine AppEngine for Java environment programmatically

Is there a way to tell programmatically at run-time whether a Google App Engine application is running locally vs. hosted? I’m looking for a way to call some custom stub code when running in a local development environment and make different calls when running hosted.

Answer

You can use com.google.appengine.api.utils.SystemProperty in AppEngine 1.3.

import com.google.appengine.api.utils.SystemProperty;
import static com.google.appengine.api.utils.SystemProperty.environment;
import static com.google.appengine.api.utils.SystemProperty.Environment.Value.Development;
import static com.google.appengine.api.utils.SystemProperty.Environment.Value.Production;

SystemProperty.Environment.Value env = environment.value();
if (env  == Production) {
      //prod only code
      ...
} else if(env == Development) {
      //dev only code
      ...
}
Categories
discuss

Why doesn’t the JVM cache JIT compiled code?

The canonical JVM implementation from Sun applies some pretty sophisticated optimization to bytecode to obtain near-native execution speeds after the code has been run a few times.

The question is, why isn’t this compiled code cached to disk for use during subsequent uses of the same function/class?

As it stands, every time a program is executed, the JIT compiler kicks in afresh, rather than using a pre-compiled version of the code. Wouldn’t adding this feature add a significant boost to the initial run time of the program, when the bytecode is essentially being interpreted?

Answer

Without resorting to cut’n’paste of the link that @MYYN posted, I suspect this is because the optimisations that the JVM performs are not static, but rather dynamic, based on the data patterns as well as code patterns. It’s likely that these data patterns will change during the application’s lifetime, rendering the cached optimisations less than optimal.

So you’d need a mechanism to establish whether than saved optimisations were still optimal, at which point you might as well just re-optimise on the fly.

Categories
discuss

how to send through ServletOutputStream characters in UTF-8 encoding

My servlet code looks like that:

response.setContentType("text/html; charset=UTF-8");
response.setCharacterEncoding("UTF-8");
ServletOutputStream out = response.getOutputStream();
out.println(...MY-UTF-8 CODE...);

then I get the error:

java.io.CharConversionException: Not an ISO 8859-1 character: ש
 javax.servlet.ServletOutputStream.print(ServletOutputStream.java:89)
 javax.servlet.ServletOutputStream.println(ServletOutputStream.java:242)
 rtm.servlets.CampaignLogicServlet.doPost(CampaignLogicServlet.java:68)
 javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
 javax.servlet.http.HttpServlet.service(HttpServlet.java:717)

How can I switch the charset of Servlet’s outputstream ???

Answer

I think you want to use getWriter() instead. That will accept a string and encode it, whereas the output stream is for handling binary data.

From the doc:

Returns a PrintWriter object that can send character text to the client. The character encoding used is the one specified in the charset= property of the setContentType(java.lang.String) method, which must be called before calling this method for the charset to take effect.

Either this method or getOutputStream() may be called to write the body, not both.

Here’s the change of the code:

response.setContentType("text/html; charset=UTF-8");
response.setCharacterEncoding("UTF-8");
PrintWriter out = response.getWriter();
out.println(...MY-UTF-8 CODE...);
Categories
discuss

Maven parent pom vs modules pom

There seem to be several ways to structure parent poms in a multiproject build and I wondering if anyone had any thoughts on what the advantages / drawbacks are in each way.

The simplest method of having a parent pom would be putting it in the root of a project i.e.

myproject/
  myproject-core/
  myproject-api/
  myproject-app/
  pom.xml

where the pom.xml is both the parent project as well as describes the -core -api and -app modules

The next method is to separate out the parent into its own subdirectory as in

myproject/
  mypoject-parent/
    pom.xml
  myproject-core/
  myproject-api/
  myproject-app/

Where the parent pom still contains the modules but they’re relative, e.g. ../myproject-core

Finally, there’s the option where the module definition and the parent are separated as in

myproject/
  mypoject-parent/
    pom.xml
  myproject-core/
  myproject-api/
  myproject-app/
  pom.xml

Where the parent pom contains any “shared” configuration (dependencyManagement, properties etc.) and the myproject/pom.xml contains the list of modules.

The intention is to be scalable to a large scale build so should be scalable to a large number of projects and artifacts.

A few bonus questions:

  • Where is the best place to define the various shared configuration as in source control, deployment directories, common plugins etc. (I’m assuming the parent but I’ve often been bitten by this and they’ve ended up in each project rather than a common one).
  • How do the maven-release plugin, hudson and nexus deal with how you set up your multi-projects (possibly a giant question, it’s more if anyone has been caught out when by how a multi-project build has been set up)?

Edit: Each of the sub projects have their own pom.xml, I’ve left it out to keep it terse.

Answer

In my opinion, to answer this question, you need to think in terms of project life cycle and version control. In other words, does the parent pom have its own life cycle i.e. can it be released separately of the other modules or not?

If the answer is yes (and this is the case of most projects that have been mentioned in the question or in comments), then the parent pom needs his own module from a VCS and from a Maven point of view and you’ll end up with something like this at the VCS level:

root
|-- parent-pom
|   |-- branches
|   |-- tags
|   `-- trunk
|       `-- pom.xml
`-- projectA
    |-- branches
    |-- tags
    `-- trunk
        |-- module1
        |   `-- pom.xml
        |-- moduleN
        |   `-- pom.xml
        `-- pom.xml

This makes the checkout a bit painful and a common way to deal with that is to use svn:externals. For example, add a trunks directory:

root
|-- parent-pom
|   |-- branches
|   |-- tags
|   `-- trunk
|       `-- pom.xml
|-- projectA
|   |-- branches
|   |-- tags
|   `-- trunk
|       |-- module1
|       |   `-- pom.xml
|       |-- moduleN
|       |   `-- pom.xml
|       `-- pom.xml
`-- trunks

With the following externals definition:

parent-pom http://host/svn/parent-pom/trunk
projectA http://host/svn/projectA/trunk

A checkout of trunks would then result in the following local structure (pattern #2):

root/
  parent-pom/
    pom.xml
  projectA/

Optionally, you can even add a pom.xml in the trunks directory:

root
|-- parent-pom
|   |-- branches
|   |-- tags
|   `-- trunk
|       `-- pom.xml
|-- projectA
|   |-- branches
|   |-- tags
|   `-- trunk
|       |-- module1
|       |   `-- pom.xml
|       |-- moduleN
|       |   `-- pom.xml
|       `-- pom.xml
`-- trunks
    `-- pom.xml

This pom.xml is a kind of “fake” pom: it is never released, it doesn’t contain a real version since this file is never released, it only contains a list of modules. With this file, a checkout would result in this structure (pattern #3):

root/
  parent-pom/
    pom.xml
  projectA/
  pom.xml

This “hack” allows to launch of a reactor build from the root after a checkout and make things even more handy. Actually, this is how I like to setup maven projects and a VCS repository for large builds: it just works, it scales well, it gives all the flexibility you may need.

If the answer is no (back to the initial question), then I think you can live with pattern #1 (do the simplest thing that could possibly work).

Now, about the bonus questions:

  • Where is the best place to define the various shared configuration as in source control, deployment directories, common plugins etc. (I’m assuming the parent but I’ve often been bitten by this and they’ve ended up in each project rather than a common one).

Honestly, I don’t know how to not give a general answer here (like “use the level at which you think it makes sense to mutualize things”). And anyway, child poms can always override inherited settings.

  • How do the maven-release plugin, hudson and nexus deal with how you set up your multi-projects (possibly a giant question, it’s more if anyone has been caught out when by how a multi-project build has been set up)?

The setup I use works well, nothing particular to mention.

Actually, I wonder how the maven-release-plugin deals with pattern #1 (especially with the <parent> section since you can’t have SNAPSHOT dependencies at release time). This sounds like a chicken or egg problem but I just can’t remember if it works and was too lazy to test it.

Categories
discuss

Find all combinations of a given set of numbers

say I have a set of numbers ‘0’, ‘1’, ‘2’, …, ‘9’. I want to find all numbers that contain exactly one of each of the numbers in my set.

The problem is: Before I start my program, I do not know how many numbers and which numbers my set will include. (For example, the set could include the numbers ‘1’, ‘3’ and ’14’.)

I searched the internet, and stumbled upon the term ‘dynamic programming’ which apparently is something to use to solve problems like mine, but I did not understand the examples.

Can somebody give me a hint on how to solve this problem (possibly with dynamic programming)?

EDIT: When the set includes numbers like ’14’ the different numbers of the set would of course have to be separated by some means, e.g. when the set includes the numbers ‘1’, ‘3’, and ’14’, combinations could be something like 1-3-14 or 3-14-1 (= individual numbers separated by a ‘-‘-character).

EDIT 2: One problem that seems to be somewhat similar is described here: one of the solutions uses dynamic programming.

Answer

To examine all the combinations without knowing in advance how many digits must have the output, I once wrote this code:

#include <stdio.h>
#include <stdlib.h>

#define ARRSIZE(arr)    (sizeof(arr)/sizeof(*(arr)))

int main()
{
    const char values[]= {'0','1','2','3','4','5','6','7','8','9','a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};
    char * buffer=NULL;
    int * stack=NULL;
    int combinationLength=-1;
    int valuesNumber=-1;
    int curPos=0;
    fprintf(stderr, "%s", "Length of a combination: ");
    if(scanf("%d", &combinationLength)!=1 || combinationLength<1)
    {
        fputs("Invalid value.n",stderr);
        return 1;
    }
    fprintf(stderr, "%s (%lu max): ", "Possible digit values",(long unsigned)ARRSIZE(values));
    if(scanf("%d", &valuesNumber)!=1 || valuesNumber<1 || (size_t)valuesNumber>ARRSIZE(values))
    {
        fputs("Invalid value.n", stderr);
        return 1;
    }
    buffer=(char *)malloc(combinationLength);
    stack=(int *)malloc(combinationLength*sizeof(*stack));
    if(buffer==NULL || stack==NULL)
    {
        fputs("Cannot allocate memory.n", stderr);
        free(buffer);
        free(stack);
        return 2;
    }
    /* Combinations generator */
    for(;;)
    {
        /* If we reached the last digit symbol... */
        if(stack[curPos]==valuesNumber)
        {
            /* ...get back to the previous position, if we finished exit */
            if(--curPos==-1)
                break;
            /* Repeat this check */
            continue;
        }
        buffer[curPos]=values[stack[curPos]];
        /* If we are in the most inner fake-cycle write the combination */
        if(curPos==combinationLength-1)
            puts(buffer);
        stack[curPos]++;
        /* If we aren't on the last position, start working on the next one */
        if(curPos<combinationLength-1)
        {
            curPos++;
            stack[curPos]=0;
        }
    }
    /* Cleanup */
    free(buffer);
    free(stack);
    return 0;    
}

It does everything just in one cycle to avoid recursion and function calls overhead, still if “fakes” the needed nested for loops using the stack array.
It performs quite well, on my 4 years old Athlon64 3800+ it takes 2′ 4″ of user time (=> actual computation time) to generate 36^6=2176782336 combinations, so it computes about 17.5 million combinations per second.

matteo@teoubuntu:~/cpp$ gcc -Wall -Wextra -ansi -pedantic -O3 combinations.c -o combinations.x
matteo@teoubuntu:~/cpp$ time ./combinations.x > /media/Dati/combinations.txt
Length of a combination: 6
Possible digit values (36 max): 36

real    13m6.685s
user    2m3.900s
sys 0m53.930s
matteo@teoubuntu:~/cpp$ head /media/Dati/combinations.txt
000000
000001
000002
000003
000004
000005
000006
000007
000008
000009
matteo@teoubuntu:~/cpp$ tail /media/Dati/combinations.txt
zzzzzq
zzzzzr
zzzzzs
zzzzzt
zzzzzu
zzzzzv
zzzzzw
zzzzzx
zzzzzy
zzzzzz
matteo@teoubuntu:~/cpp$ ls -lh /media/Dati/combinations.txt 
-rwxrwxrwx 1 root root 15G 2010-01-02 14:16 /media/Dati/combinations.txt
matteo@teoubuntu:~/cpp$ 

The “real” time is quite high because I was also doing something else on the PC in the meanwhile.

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