CRC32 calculation on Java and Objective-C

One time I had to calculate CRC32 checksum for image files on Java and Objective-C. But I could not get the results matched when I tried to read the file via UIImagePickerController on Objective-C. I knew the result from Java was correct because this website returned the same checksum. The myth was finally resolved when I loaded the file from resource by using NSBundle.

Source code

code snippet in Java

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.zip.CRC32;

...
try {
    Path path = Paths.get("path/to/image.jpg");
    byte[] data = Files.readAllBytes(path);
    Checksum checksum = new CRC32();
    checksum.update(data, 0, data.length);
    long result = checksum.getValue();
    System.out.println("file crc32 cheksum: " + result);
} catch (IOException e){
    e.printStackTrace();
}

code snippet in Objective-C

you will need to add libz.dylib in Linked Frameworks and Libraries in General tab on Xcode.

#include <zlib.h>;

...
NSString *filePath = [[NSBundle mainBundle] pathForResource:@"image" ofType:@"jpg"];
NSData *data = [NSData dataWithContentsOfFile:filePath];
uLong crc = crc32(0L, Z_NULL, 0);
long long result = crc32(crc, [data bytes], [data length]);
NSLog(@"file crc32 checksum %llu", result));

Create executable jar on Eclipse

1. After the coding is complete, run the project as Java Application. This step is mandatory for Eclipse to create the Launch configuration info.

2. Identify the Launch configuration name by going to "Run Configurations..." (right click on project -> Run As). On the "Main" tab, there is a Name textbox on the top. The value is editable.

3. Export the project as "Runnable JAR file" (Right click on project -> Export). On the next step, select the Launch configuration for the name and set the Export destination.

4. After the jar is generated, run the application by java -jar <app_name.jar>

The Hello World JNI example

A simple example to use native code (C/C++ in this tutorial) via JNI (Java Native Interface). The Java program will take the input from user, and then pass the value to native code to print out on console.

Setup

• Linux Mint - a modern, elegant and comfortable Linux distro
• OpenJDK - the JDK on Linux Mint
• gcc - the c compiler

Installation

Make usre the software is installed properly

$ javac -version
javac 1.7.0_75
$ gcc --version
gcc (Ubuntu 4.8.2-19ubuntu1) 4.8.2
...

Development

Demo.java

package com.example.jni;

public class Demo {

    // load native library at runtime
    static {
        System.loadLibrary("myNative");
    }

    // declare a native method
    public native void helloNative(String message);

    public static void main(String[] args) {
        new Demo().helloNative(args[0]);
    }
}

generate the header file com_example_jni_Demo.h

path/to/java/src $ javah com.example.jni.Demo

no need to modify the header file, but it declares the method to implement

JNIEXPORT void JNICALL Java_com_example_jni_Demo_helloNative
  (JNIEnv *, jobject, jstring);

myNative.c

#include 
#include "com_example_jni_Demo.h"

JNIEXPORT void JNICALL Java_com_example_jni_Demo_helloNative(JNIEnv *env, jobject obj, jstring javaString)
{
    const char *message;
    message = (*env)->GetStringUTFChars(env, javaString, 0);
    printf("native echo: %s\n", message);
}

generate the Shared Library which will be libmyNative.so

path/to/java/src $ gcc -shared -fPIC -o libmyNative.so -I /usr/lib/jvm/java-7-openjdk-amd64/include -I /usr/lib/jvm/java-7-openjdk-amd64/include/linux myNative.cdino

compile and run the program

path/to/java/src $ javac com/example/jni/Demo.java
path/to/java/src $ java -Djava.library.path=path/to/java/src Demo

To run the Java program on Eclipse, add -Djava.library.path=path/to/shared/library to Run configurations -> Arguments -> VM arguments

Resolve Element 'mvc:annotation-driven' must have no character

To resolve the error

cvc-complex-type.2.1: Element 'mvc:annotation-driven' must have no character or element information item [children], because the type's content type is empty.

chagne the mvc schemaLocation to newer version, for example, 3.2 works for me.

<beans xmlns="http://www.springframework.org/schema/beans"
        ...
 xsi:schemaLocation="
        http://www.springframework.org/schema/beans     
        ...
        http://www.springframework.org/schema/mvc
        http://www.springframework.org/schema/mvc/spring-mvc-3.2.xsd">

Compute SHA checksum for a string on MAC terminal

$echo -n <the_string> | shasum -a 256

Explanation:
The option -n tells echo not to output the trailing newline.
The option -a 256 of shasum specifies to use 256 algorithm.

MongoDB error E11000 duplicate key error index on Spring

When trying to insert the document with the same id on MongoDB, MongoDB will return an error of E11000 duplicate key error index.

> db.users.insert({"_id":"jsmith", "firstname": "John", "lastname": "Smith"})
> db.users.insert({"_id":"jsmith", "firstname": "John", "lastname": "Smith"})
E11000 duplicate key error index: database.users.$_id_  dup key: { : "jsmith" }

But you might have chance to be surprised that the MongoOperations(more precisely, org.springframework.data.mongodb.core.MongoOperations) does not complain even you insert the same object twice in Spring.

As of today, the latest release of spring-data-mongodb

<dependency>
    <groupId>org.springframework.data</groupId>
    <artifactId>spring-data-mongodb</artifactId>
    <version>1.4.0.RELEASE</version>
</dependency>

The POJO class, User.java

public class User {
 
    @Id
    private String id;
    private String firstname;
    private String lastname;

    public User(String id, String username, String password) {
        super();
        this.id = id;
        this.username = username;
        this.password = password;
    }

    getters and setters
    ...
}

The main function

User user = new User("jsmith", "John", "Smith");
mongoOps.insert(user);
User user = new User("jsmith", "John", "Smith");
mongoOps.insert(user);

The code will run with no exception, but only one entry will be inserted. In order to see the "E11000 duplicate key error index" error in Spring, configure mongo with write-concern, e.g. <mongo:mongo host="localhost" port="27017" write-concern="SAFE" />

Then you will see org.springframework.dao.DuplicateKeyException is thrown.

Reference
Where's My Exceptions, Spring Data MongoDB?
MongoDB: No exception when saving duplicate value to an attribute annotated with @Indexed(unique=true)

Spring MVC and Cookie

In a common scenario, a cookie may be set on the client side using JavaScript

function setCookie() {
    var c_name = "javascriptCookie";
    var c_value = "this cookie was set by JavaScript";
    document.cookie=c_name + "=" + c_value;
}

Spring MVC can also write cookie while returning a page

@RequestMapping(value="/setcookie", method = RequestMethod.GET)
public String setCookie(HttpServletResponse response){
    response.addCookie(new Cookie("srpingCookie", "this cookie was set by Spring"));
    return "cookieIsSet";
}

To read cookies on Spring

// all the cookies can be seen here regardless of that the cookie was set on client side or server side
@RequestMapping(value="/readcookie", method = RequestMethod.GET)
public String showCookie(HttpServletRequest request) {
    Cookie[] cookies = request.getCookies();
    if (cookies != null){
 for (Cookie cookie : cookies){
     System.out.println(cookie.getName() + " : " + cookie.getValue());
        }
    }
        
    ...
}

or

// mycookie must be available otherwise HTTP 400 occurs
// the cookie value will be unescape'd automatically
@RequestMapping(value="/readparticularcookie", method = RequestMethod.GET)
public String showDinoCookie(@CookieValue("mycookie") String cookie){
    System.out.println(cookie);

    ...
}

Reference
http://www.w3schools.com/js/js_cookies.asp