Google App Engine Devserver Exception Due to FormatStyle Restricted Class

In one of my application in Google App Engine, I tried to integrated Spring MVC.

The Spring version I used was 4.0.13.

Unfortunately, after the integration, it started to throw series of Exceptions following type one after another.

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping#0': Initialization of bean failed [...]

nested exception is java.lang.NoClassDefFoundError: java.time.format.FormatStyle is a restricted class. Please see the Google App Engine developer's guide for more details.

Problem and Solution

The problem was, I had been using Java 8. With the integration of Spring, Spring Libraries try to use some of the classes which are restricted in App Engine (why?). So if I downgrade Java 1.8 to 1.7 then the problem will be fixed. However this cannot be the solution as anyone wants to use the latest version of Java.

The actual problem is, even though I had been using Java 8, App Engine's target JRE was still 1.7. So to fix this problem I had to add following snippet into appengine-web.xml:

<runtime>java8</runtime>

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Java String.replaceAll() -- replace only the Matched String not the REGEX

Every Java programmer must have used the method,

String.replaceAll();

Usage Example

    "this is a test and that is another test".replaceAll("test", "rule");

the output of this execution will be:

"this is a rule and that is another rule"

Twist!!!

The usage of the method seems to be straightforward. However its not.

The first parameter to the method is actually a regular expression. You may fall into it, if the first argument has some special character which has different meaning in REGEX world.

What if you still want to match the expression as whole instead of as regex? You will need to convert it into String literal. You can do this by placing your expression in between Regex Quoted Literals. i.e. \Q <param> \E.

Example:

String test = "<span class=\"colorful\">This is a colorful text.</span>";
String replace_start = "<span class=\"colorful\">";
String replace_end = "</span>";

String expectedOutput = "This is a colorful text.";

Without Quoted Literals:

    String result = test.replace(replace_start, "").replace(replace_end, "");
    Assert.assertEquals(expectedOutput, result);

// This will fail because result = "<span class=\"colorful\">This is a colorful text.</span>"

With Quoted Literals

    String result = test.replace("\\Q" + replace_start + "\\E", "").replace("\\Q" + replace_end + "\\E", "");
    Assert.assertEquals(expectedOutput, result);

This will pass.

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Get List of Links of All Shared Files From Your Dropbox (Using Java API)

Recently I had to extract the links of all the files shared in my dropbox.

Following was the first approach I used.

public List getLinks(DbxClientV2 client) throws DbxException {
    List result = new ArrayList();

    ListSharedLinksResult sharedLinksResult = client.sharing().listSharedLinks();
    for (SharedLinkMetadata slm : sharedLinksResult.getLinks()) {
      result.add(slm.getUrl());
    }

    return result;
  }

At first sight, this method seems to work perfectly fine. However, there is a glitch in the approach. The method

ListSharedLinksResult sharedLinksResult = client.sharing().listSharedLinks();

actually doesn't return all the result. It returns few items at a time. We can know whether there are more items or not by using

sharedLinksResult.getHasMore();

which returns True if there are more items left, False otherwise.

So if there are more items left, then we need to extract them as well.

sharedLinksResult has a cursor, which can be used to extract additional items. This can be done using:

client.sharing().listSharedLinks(new ListSharedLinksArg((String)null, sharedLinksResult.getCursor(), true));

But its not as easy as it looks. The problem is that the overloaded method listSharedLinks(ListSharedLinksArg) is not a Public Method. So you cannot you use just anywhere in your code.

Hack!!!

I managed to use this, by placing my class into the same package where the class DbxUserSharingRequests lie i.e. com.dropbox.core.v2. Obviously, this is not a clean approach.

Solution

As it turns out, the solution is to use

DbxUserSharingRequests.listSharedLinksBuilder()

where we can set the Cursor from sharedLinksResult. Following is my final solution:

public List getLinks(DbxClientV2 client) throws DbxException {
    List result = new ArrayList();

    ListSharedLinksResult sharedLinksResult = client.sharing().listSharedLinks();

    while(true) {
      for (SharedLinkMetadata slm : sharedLinksResult.getLinks()) {
        result.add(slm.getUrl());
      }

      if (sharedLinksResult.getHasMore()) {
        sharedLinksResult = client.sharing().listSharedLinksBuilder().withCursor(sharedLinksResult.getCursor()).withDirectOnly(true).start();
      } else {
        break;
      }
    }
    return result;
  }

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Commonly Used Vi Vim Commands

REFERENCE: https://manandharsabbirk.appspot.com/articles/populate_article.html?id=mjdr45v1gsfwzyc/vi_common_commands.html

Show Line Numbers

:set number

This command is used to displaying line numbers in the editor.

:set nonumber

This is the counter command to the first numer. That is, this command will hide the line numbers.

Auto Indent

:autoindent

This command will auto indent the line as the previous line.
At any time, if you need to remove the indentation of the line you can press Ctrl+d.

Switching to New File

If you are working on a file in VIM and want to switch to a new file then you can use the following command:

:e    <file_name>

Split Screen

Sometimes we want to compare 2 files or the segments of same file. At such time, split feature of vim comes in handy. Screen can be easily split in vim. 

Split Screen Horizontally

:split

This code will split the screen horizontally. You can pass optional filename, if you want to open new file in the new window. If you don't pass any value, then the same file will be opened.

Split Screen Vertically

:vsplit

This code will split the screen vertically. Similar, to horizontal split, you can also pass a filename to open in the new window. 

Navigating the Splitted Screens

You can navigate the splitted screens by pressing Ctrl+w TWICE.
You can also do this by pressing Ctrl+w and then using the ARROW keys.

Close / Unsplit the Splitted Screens

CLICK HERE

Move to the End of Line

$

Move to the Last Line

:$

Move to a Line Number

:<line_number>

Note: :$ will move to last line.

Delete All

:1,$d

Copy/Paste Lines

yy

This command will copy the current line.

<number>yy

This command will copy <number> lines starting from the current line.

p

or

P

This command will paste the copied text.

Cut/Paste Lines

dd

This command will cut the current line.

<number>dd

This command will cut <number> lines starting from the current line.

p

or

P

This command will paste the copied text.

Copy/Cut Paste Selected Text

1. Place the cursor to the position from where the text is to copied.
2. Press v to begin the text selction.
3. Move cursor to select the text.
4. Press y to COPY or d to CUT the selected text.
5. Press p or P to paste the text.

Insert Text (Spaces) in Multiple Lines at once

1. Within VIM select block visual mode, by pressing CTRL-V .
2. Use the cursor keys to select the lines you want to add the spaces to.
3. Then enter :'<,'> norm I
4. This will then insert a single space to the beginning of your select lines.

Lets break down the command :'<,'> norm I

1. :'<,'> : For the line I selected
2. norm : Execute the following sequence of keystrokes as if I was in normal mode
3. I: Insert at the beginning of the line the following characters
4. [space]: The character(s) you would like to insert

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Generating Pagination Numbers

Very often we need to generate series of numbers for pagination. For small number of pages, all the page numbers can be displayed. However, when there are large number of pages, then we need to display only appropriate page numbers.

Below is a Java Code that can be used to generate the sequence.

It can be easily converted into Javascript function and make necessary changes as required to display in your web site.

public class Pagination {

     public static void main(String []args){
        generatePaginationNumbers(150, 5, 13);
     }
     
     public static void generatePaginationNumbers(int totalItems, int pageSize, int currentPage) {
         int DELTA = 3;
         int SHOW_ALL_SIZE = 10;
         int lastPage = totalItems/pageSize;
         if (totalItems > lastPage * pageSize) {
             lastPage++;
         }
         
         currentPage = currentPage < 1 ? 1 : currentPage;
         if (currentPage > lastPage) {
             currentPage = 1;
         }
         
         if (totalItems < 1) {
             return;
         }
         
         if (currentPage > 1) {
             System.out.print("prev ");
         }
         printPaginationPage(1, currentPage);
         if (totalItems > pageSize) {
             printPaginationPage(2, currentPage);
         } else {
             return;
         }
         
         if (currentPage - DELTA > 3 && lastPage > SHOW_ALL_SIZE) {
             System.out.print(" ... ");
         }
         
         int page = lastPage > SHOW_ALL_SIZE ? currentPage - DELTA : 3;
         page = page <= 2 ? 3 : page;
         int end = currentPage + DELTA;
         end = end >= lastPage - 1 ? lastPage - 2 : end;
         for( ; page <= end; page++) {
             printPaginationPage(page, currentPage);
         }
         
         if (page <= lastPage - 2 && lastPage > SHOW_ALL_SIZE) {
             System.out.print(" ... ");
         }
         
         printPaginationPage(lastPage - 1, currentPage);
         printPaginationPage(lastPage, currentPage);
         
         if (currentPage < lastPage) {
             System.out.print("next");
         }
     }
     
     public static void printPaginationPage(int page, int current) {
         if (page == current) {
             System.out.print("[" + page + "] ");
         } else {
             System.out.print(page + " ");
         }
     }
}

Sample Inputs and Outputs

generatePaginationNumbers(150, 5, 13);
Output: prev 1 2  ... 10 11 12 [13] 14 15 16  ... 29 30 next

generatePaginationNumbers(150, 5, 28);
Output: prev 1 2  ... 25 26 27 [28] 29 30 next

generatePaginationNumbers(150, 5, 2);
Output: prev 1 [2] 3 4 5  ... 29 30 next

generatePaginationNumbers(150, 5, 1);
Output: [1] 2 3 4  ... 29 30 next

generatePaginationNumbers(150, 5, 30);
Output: prev 1 2  ... 27 28 29 [30]

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Harry Potter Audio

Are you the fan of Harry Potter?

Do you want to listen to audios of Harry Potter Series?

Here are two easy options for you

1. DOWNLOAD JAR file (This is to be run with Java 8 Update 121 or above)



2. DOWNLOAD APK file to run in your Android Device

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Too Many Redirect with SSL in NGINX and Jetty Server

Recently I faced a very nasty problem.

I am using Nginx as my web server and Java Web Application as App Server hosted in Jetty.

I was required to host all the pages using SSL i.e. HTTPS. I enabled Nginx to handle the HTTPS requests at the port 443. Nginx did handle the https requests however Jetty server received all the HTTPS requests as HTTP. There I had the logic to redirect all HTTP to HTTPS. So I fell into the problem that it was redirecting again and again, therefore causing Too Many Redirect error.

I had to spent a lot of time and finally I found the error and hence solution.

While the problem was HTTPS scheme was getting lost at load balancer. This could be solved using the following snippet into the web.xml

  <context-param>
    <param-name>org.mortbay.jetty.servlet.SessionPath</param-name>
    <param-value>/</param-value>
  </context-param>

  <filter>
    <filter-name>XForwardedFilter</filter-name>
    <filter-class>fr.xebia.servlet.filter.XForwardedFilter</filter-class>
    <init-param>
      <param-name>protocolHeader</param-name>
      <param-value>x-forwarded-proto</param-value>
    </init-param>
  </filter>

  <filter-mapping>
    <filter-name>XForwardedFilter</filter-name>
    <url-pattern>/*</url-pattern>
    <dispatcher>REQUEST</dispatcher>
  </filter-mapping>

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Best Loop Idiom for special casing the last element

References: StackOverflow1 StackOverflow2

Very often it happens that you need to iterate through the elements of a list/array and treat the first/last element in a special way.

For example: if you have an array {"tom", "jerry", "dog"} and want to print in a comma separated fashion.

Normally I end up appending Comma (",") after each element and finally after the loop exits, I just delete the last element which is unnecessary comma.

Turns out there are few elegant way to achieve this. One of the way is:

Iterator itemIterator = Arrays.asList(items).iterator();
if (itemIterator.hasNext()) {
  // special-case first item.  in this case, no comma
  while (itemIterator.hasNext()) {
    // process the rest
  }
}

This method is in fact used by Joiner class in Google Collections.

Complexity of Arrays.asList(items)

My initial suspicion was on Arrays.asList() method. I thought that the complexity of this method was O(n).

However the time complexity of this method is actually O(1) because list is merely a wrapper around the array.

Java API Documentation: Returns a fixed-size list backed by the specified array. (Changes to the returned list "write through" to the array.)

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