Implementing an Android Content Provider

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As discussed in the previous chapter, content providers provide a mechanism through which the data stored by one Android application can be made accessible to other applications. Having provided a theoretical overview of content providers, this chapter will continue the coverage of content providers by extending the Database project created in the chapter entitled An Android SQLite Database Tutorial to implement content provider based access to the database.

In order to keep the original Database project intact, we will make a copy of the project and modify that copy to implement content provider support for the application. A copy of the project may be created by right-clicking on the Database entry in the Package Explorer panel and selecting the Copy option from the resulting menu. Right-click on any white space area within the Package Explorer panel, this time selecting the Paste menu option. This will result in the appearance of the Copy Project dialog within which the project name should be entered as DatabaseProvider before the OK button is clicked.

At this point, an exact copy of the Database project named DatabaseProvider should be listed in the Package Explorer window. From this point on all changes outlined in this chapter are to be performed in the DatabaseProvider project.

The next step is to add a new package to the DatabaseProvider project into which the content provider class will be created. Add this new package by right clicking on the DatabaseProvider entry in the Package Explorer panel and selecting the New -> Package menu option. In the New Java Package dialog, enter the following package name into the Name: field before clicking on the Finish button:

com.example.database.provider

The new package should now be listed under the src folder in the DatabaseProvider section of the Package Explorer panel as illustrated in Figure 36-1:

Figure 36-1

As discussed in Understanding Android Content Providers, content providers are created by subclassing the android.content.ContentProvider class. Consequently, the next step is to add a class to the new com.example.database.provider package to serve as the content provider for this application. Locate the new package in the Package Explorer panel, right-click on it and select the New -> Class menu option. In the New Java Class dialog, verify that com.example.database.provider is shown in the Package: field and enter ContentProvider into the Name field.

Click on the Browse… button to the right of the Superclass field and enter ContentProvider into the Choose a type: text field. From the list of matching results, double click on the ContentProvider – android.content class.

On returning to the New Java Class dialog, click on the Finish button to create the new class. Once the new class has been created, the ContentProvider.java file should be listed beneath the com.example.database.provider package in the Package Explorer panel. Double click on this file to view the source code, which should appear as outlined in the following listing:

package com.example.database.provider;

import android.content.ContentValues;
import android.database.Cursor;
import android.net.Uri;

public class ContentProvider extends android.content.ContentProvider {

	@Override
	public int delete(Uri arg0, String arg1, String[] arg2) {
		// TODO Auto-generated method stub
		return 0;
	}

	@Override
	public String getType(Uri arg0) {
		// TODO Auto-generated method stub
		return null;
	}

	@Override
	public Uri insert(Uri arg0, ContentValues arg1) {
		// TODO Auto-generated method stub
		return null;
	}

	@Override
	public boolean onCreate() {
		// TODO Auto-generated method stub
		return false;
	}

	@Override
	public Cursor query(Uri arg0, String[] arg1, String arg2, 
                 String[] arg3, String arg4) {
		// TODO Auto-generated method stub
		return null;
	}

	@Override
	public int update(Uri arg0, ContentValues arg1, String arg2,
                 String[] arg3) {
		// TODO Auto-generated method stub
		return 0;
	}

}

As is evident from a quick review of the code in this file, Eclipse has already populated the class with stubs for each of the methods that a subclass of ContentProvider is required to implement. It will soon be necessary to begin implementing these methods, but first some constants relating to the provider’s content authority and URI need to be declared.

As outlined in the previous chapter, all content providers must have associated with them an authority and a content uri. In practice, the authority is typically the full package name of the content provider class itself, in this case com.example.database.provider.ContentProvider.

The content URI will vary depending on application requirements, but for the purposes of this example will comprise the authority with the name of the database table appended at the end. Within the ContentProvider.java file, make the following modifications:

package com.example.database.provider;

import android.content.ContentValues;
import android.database.Cursor;
import android.net.Uri;
import android.content.UriMatcher;

public class ContentProvider extends android.content.ContentProvider {

	private static final String AUTHORITY = 
                "com.example.database.provider.ContentProvider";
	private static final String PRODUCTS_TABLE = "products";
	public static final Uri CONTENT_URI = Uri.parse("content://" + AUTHORITY + "/" + PRODUCTS_TABLE);
.
.
.
}

The above statements begin by creating a new String object named AUTHORITY and assigning the authority string to it. Similarly, a second String object named PRODUCTS_TABLE is created and initialized with the name of our database table (products).

Finally, these two string elements are combined, prefixed with content:// and converted to a Uri object using the parse() method of the Uri class. The result is assigned to a variable named CONTENT_URI.

When the methods of the content provider are called, they will be passed as an argument a URI indicating the data on which the operation is to be performed. This URI may take the form of a reference to a specific row in a specific table. It is also possible that the URI will be more general, for example specifying only the database table. It is the responsibility of each method to identify the Uri type and to act accordingly. This task can be eased considerably by making use of a UriMatcher instance. Once a UriMatcher instance has been created, it can be configured to return a specific integer value corresponding to the type of URI it detects when asked to do so. For the purposes of this tutorial, we will be configuring our UriMatcher instance to return a value of 1 when the URI references the entire products table, and a value of 2 when the URI references the ID of a specific row in the products table. Before working on creating the URIMatcher instance, we will first create two integer variables to represent the two URI types:

package com.example.database.provider;

import android.content.ContentValues;
import android.database.Cursor;
import android.net.Uri;
import android.content.UriMatcher;

public class ContentProvider extends android.content.ContentProvider {

	private static final String AUTHORITY = 
             "com.example.database.provider.ContentProvider";
	private static final String PRODUCTS_TABLE = "products";
	public static final Uri CONTENT_URI = Uri.parse("content://" + AUTHORITY + "/" + PRODUCTS_TABLE);
	
	public static final int PRODUCTS = 1;
	public static final int PRODUCTS_ID = 2;
.
.
}

With the Uri type variables declared, it is now time to add code to create a UriMatcher instance and configure it to return the appropriate variables:

public class ContentProvider extends android.content.ContentProvider {

	private static final String AUTHORITY = 
                  "com.example.database.provider.ContentProvider";
	private static final String PRODUCTS_TABLE = "products";
	public static final Uri CONTENT_URI = Uri.parse("content://" 
                  + AUTHORITY + "/" + PRODUCTS_TABLE);
	
	public static final int PRODUCTS = 1;
	public static final int PRODUCTS_ID = 2;
	
	private static final UriMatcher sURIMatcher = 
                              new UriMatcher(UriMatcher.NO_MATCH);
	
	static {
	    sURIMatcher.addURI(AUTHORITY, PRODUCTS_TABLE, PRODUCTS);
	    sURIMatcher.addURI(AUTHORITY, PRODUCTS_TABLE + "/#", 
                                          PRODUCTS_ID);
	}
.
.
}

The UriMatcher instance (named sURIMatcher) is now primed to return the value of PRODUCTS when just the products table is referenced in a URI, and PRODUCTS_ID when the URI includes the ID of a specific row in the table.

When the content provider class is created and initialized, a call will be made to the onCreate() method of the class. It is within this method that any initialization tasks for the class need to be performed. For the purposes of this example, all that needs to be performed is for an instance of the MyDBHandler class implemented in An Android SQLite Database Tutorial to be created. Once this instance has been created, it will need to be accessible from the other methods in the class, so a declaration for the database handler also needs to be declared, resulting in the following code changes to the ContentProvider.java file:

package com.example.database.provider;

import com.example.database.MyDBHandler;

import android.content.ContentValues;
import android.database.Cursor;
import android.net.Uri;
import android.content.UriMatcher;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteQueryBuilder;
import android.text.TextUtils;

public class ContentProvider extends android.content.ContentProvider {

	private MyDBHandler myDB;
	
	private static final String AUTHORITY = 
                "com.example.database.provider.ContentProvider";
	private static final String PRODUCTS_TABLE = "products";
	public static final Uri CONTENT_URI = Uri.parse("content://" + AUTHORITY
	        + "/" + PRODUCTS_TABLE);
	
	public static final int PRODUCTS = 1;
	public static final int PRODUCTS_ID = 2;
.
.
.
	@Override
	public boolean onCreate() {
		myDB = new MyDBHandler(getContext(), null, null, 1);
		return false;
	}
}

When a client application or activity requests that data be inserted into the underlying database, the insert() method of the content provider class will be called. At this point, however, all that exists in the ContentProvider.java file of the project is a stub method, which reads as follows:

@Override
public Uri insert(Uri arg0, ContentValues arg1) {
	// TODO Auto-generated method stub
	return null;
}

Passed as arguments to the method are a URI specifying the destination of the insertion and a ContentValues object containing the data to be inserted.

This method now needs to be modified to perform the following tasks:

• Use the sUriMatcher to identify the URI type.
• Throw an exception if the URI is not valid.
• Obtain a reference to a writable instance of the underlying SQLite database.
• Perform a SQL insert operation to insert the data into the database table.
• Notify the corresponding content resolver that the database has been modified.
• Return the URI of the newly added table row.

Bringing these requirements together results in a modified insert() method, which reads as follows (note also that the argument names have been changed from arg0 and arg1 to names that are more self-explanatory):

@Override
public Uri insert(Uri uri, ContentValues values) {
	int uriType = sURIMatcher.match(uri);
		   
	SQLiteDatabase sqlDB = myDB.getWritableDatabase();

	long id = 0;
	switch (uriType) {
		case PRODUCTS:
		 id = sqlDB.insert(MyDBHandler.TABLE_PRODUCTS, null, values);
		break;
		default:
		 throw new IllegalArgumentException("Unknown URI: " + uri);
	}
	getContext().getContentResolver().notifyChange(uri, null);
	return Uri.parse(PRODUCTS_TABLE + "/" + id);
}

When a content provider is called upon to return data, the query() method of the provider class will be called. When called, this method is passed some or all of the following arguments:

• URI – The URI specifying the data source on which the query is to be performed. This can take the form of a general query with multiple results, or a specific query targeting the ID of a single table row.

• Projection – A row within a database table can comprise multiple columns of data. In the case of this application, for example, these correspond to the ID, product name and product quantity. The projection argument is simply a String array containing the name for each of the columns that is to be returned in the result data set.

• Selection – The “where” element of the selection to be performed as part of the query. This argument controls which rows are selected from the specified database. For example, if the query was required to select only products named “Cat Food” then the selection string passed to the query() method would read productname = “Cat Food”.

• Selection Args – Any additional arguments that need to be passed to the SQL query operation to perform the selection.

• Sort Order' – The sort order for the selected rows.

When called, the query() method is required to perform the following operations:

• Use the sUriMatcher to identify the Uri type.
• Throw an exception if the URI is not valid.
• Construct a SQL query based on the criteria passed to the method. For convenience, the SQLiteQueryBuilder class can be used in construction of the query.
• Execute the query operation on the database.
• Notify the content resolver of the operation.
• Return a Cursor object containing the results of the query.

With these requirements in mind, the code for the query() method in the ContentProvider.java file should now read as outlined in the following listing:

@Override
public Cursor query(Uri uri, String[] projection, String selection,
	        String[] selectionArgs, String sortOrder) {
		
	SQLiteQueryBuilder queryBuilder = new SQLiteQueryBuilder();
	queryBuilder.setTables(MyDBHandler.TABLE_PRODUCTS);

	int uriType = sURIMatcher.match(uri);

	switch (uriType) {
	    case PRODUCTS_ID:
	        queryBuilder.appendWhere(MyDBHandler.COLUMN_ID + "="
	                + uri.getLastPathSegment());
	        break;
	    case PRODUCTS:
	        break;
	    default:
	        throw new IllegalArgumentException("Unknown URI");
	}

	Cursor cursor = queryBuilder.query(myDB.getReadableDatabase(),
	     projection, selection, selectionArgs, null, null, sortOrder);
	cursor.setNotificationUri(getContext().getContentResolver(), uri);
	return cursor;
} 

The update() method of the content provider is called when changes are being requested to existing database table rows. The method is passed a URI, the new values in the form of a ContentValues object and the usual selection argument strings.

When called, the update() method would typically perform the following steps:

• Use the sUriMatcher to identify the URI type.
• Throw an exception if the URI is not valid.
• Obtain a reference to a writable instance of the underlying SQLite database.
• Perform the appropriate update operation on the database depending on the selection criteria and the URI type.
• Notify the content resolver of the database change.
• Return a count of the number of rows that were changed as a result of the update operation.

A general-purpose update() method, and the one we will use for this project, would read as follows:

public int update(Uri uri, ContentValues values, String selection,
		      String[] selectionArgs) {

		int uriType = sURIMatcher.match(uri);
		SQLiteDatabase sqlDB = myDB.getWritableDatabase();
		int rowsUpdated = 0;

		switch (uriType) {
		  case PRODUCTS:
		    rowsUpdated = sqlDB.update(MyDBHandler.TABLE_PRODUCTS, 
		        values, 
		        selection,
		        selectionArgs);
		    break;
		  case PRODUCTS_ID:
		    String id = uri.getLastPathSegment();
		    if (TextUtils.isEmpty(selection)) {
		      rowsUpdated = sqlDB.update(MyDBHandler.TABLE_PRODUCTS, 
		          values,
		          MyDBHandler.COLUMN_ID + "=" + id, 
		          null);
		    } else {
		      rowsUpdated = sqlDB.update(MyDBHandler.TABLE_PRODUCTS, 
		          values,
		          MyDBHandler.COLUMN_ID + "=" + id 
		          + " and " 
		          + selection,
		          selectionArgs);
		    }
		    break;
		  default:
		    throw new IllegalArgumentException("Unknown URI: " + uri);
		  }
		getContext().getContentResolver().notifyChange(uri, null);
	      return rowsUpdated;
}

The update() method of the content provider is called when changes are being requested to existing database table rows. The method is passed a URI, the new values in the form of a ContentValues object and the usual selection argument strings.

When called, the update() method would typically perform the following steps:

• Use the sUriMatcher to identify the URI type.
• Throw an exception if the URI is not valid.
• Obtain a reference to a writable instance of the underlying SQLite database.
• Perform the appropriate update operation on the database depending on the selection criteria and the URI type.
• Notify the content resolver of the database change.
• Return a count of the number of rows that were changed as a result of the update operation.

A general-purpose update() method, and the one we will use for this project, would read as follows:

public int update(Uri uri, ContentValues values, String selection,
		      String[] selectionArgs) {

		int uriType = sURIMatcher.match(uri);
		SQLiteDatabase sqlDB = myDB.getWritableDatabase();
		int rowsUpdated = 0;

		switch (uriType) {
		  case PRODUCTS:
		    rowsUpdated = sqlDB.update(MyDBHandler.TABLE_PRODUCTS, 
		        values, 
		        selection,
		        selectionArgs);
		    break;
		  case PRODUCTS_ID:
		    String id = uri.getLastPathSegment();
		    if (TextUtils.isEmpty(selection)) {
		      rowsUpdated = sqlDB.update(MyDBHandler.TABLE_PRODUCTS, 
		          values,
		          MyDBHandler.COLUMN_ID + "=" + id, 
		          null);
		    } else {
		      rowsUpdated = sqlDB.update(MyDBHandler.TABLE_PRODUCTS, 
		          values,
		          MyDBHandler.COLUMN_ID + "=" + id 
		          + " and " 
		          + selection,
		          selectionArgs);
		    }
		    break;
		  default:
		    throw new IllegalArgumentException("Unknown URI: " + uri);
		  }
		getContext().getContentResolver().notifyChange(uri, null);
	      return rowsUpdated;
}

In common with a number of other content provider methods, the delete() method is passed a URI, a selection string and an optional set of selection arguments. A typical delete() method will also perform the following, and by now largely familiar, tasks when called:

• Use the sUriMatcher to identify the URI type.
• Throw an exception if the URI is not valid.
• Obtain a reference to a writable instance of the underlying SQLite database.
• Perform the appropriate delete operation on the database depending on the selection criteria and the Uri type.
• Notify the content resolver of the database change.
• Return the number of rows deleted as a result of the operation.

A typical delete() method is, in many ways, very similar to the update() method and may be implemented as follows:

public int delete(Uri uri, String selection, String[] selectionArgs) {	

	int uriType = sURIMatcher.match(uri);
	SQLiteDatabase sqlDB = myDB.getWritableDatabase();
	int rowsDeleted = 0;

	switch (uriType) {
	    case PRODUCTS:
	      rowsDeleted = sqlDB.delete(MyDBHandler.TABLE_PRODUCTS,
              selection,
	        selectionArgs);
	        break;
	      
	    case PRODUCTS_ID:
	      String id = uri.getLastPathSegment();
	      if (TextUtils.isEmpty(selection)) {
	        rowsDeleted = sqlDB.delete(MyDBHandler.TABLE_PRODUCTS,
	        		MyDBHandler.COLUMN_ID + "=" + id, 
	            null);
	      } else {
	        rowsDeleted = sqlDB.delete(MyDBHandler.TABLE_PRODUCTS,
	        		MyDBHandler.COLUMN_ID + "=" + id 
	            + " and " + selection,
	            selectionArgs);
	      }
	      break;
	    default:
	      throw new IllegalArgumentException("Unknown URI: " + uri);
	    }
	    getContext().getContentResolver().notifyChange(uri, null);
	    return rowsDeleted;
}

With these methods implemented, the content provider class, in terms of the requirements for this example at least, is complete. The next step is to make sure that the content provider is declared in the project manifest file so that it is visible to any content resolvers seeking access to it.

Unless a content provider is declared in the manifest file of the application to which it belongs, it will not be possible for a content resolver to locate and access it. As outlined, content providers are declared using the <provider> tag and the manifest entry must correctly reference the content provider authority and content URI.

For the purposes of this project, therefore, locate the AndroidManifest.xml file for the DatabaseProvider project within the Package Explorer and double click on it to load it into the editing panel. Within the editing panel, select the AndroidManifest.xml tab along the bottom edge of the editor to modify the XML structure directly so that it reads as follows:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.database"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="1" />

    <application
      android:allowBackup="true"
      android:icon="@drawable/ic_launcher"
      android:label="@string/app_name"
      android:theme="@style/AppTheme" >
      <activity
          android:name="com.example.database.DatabaseActivity"
          android:label="@string/app_name" >
          <intent-filter>
              <action android:name="android.intent.action.MAIN" />
              <category android:name="android.intent.category.LAUNCHER" />
          </intent-filter>
      </activity>
      <provider
       android:name="com.example.database.provider.ContentProvider" 
       android:authorities="com.example.database.provider.ContentProvider" 
      />
    </application>
</manifest>

All that remains before testing the application is to modify the database handler class to use the content provider instead of directly accessing the database.

When this application was originally created, it was designed to use a database handler to access the underlying database directly. Now that a content provider has been implemented, the database handler needs to be modified so that all database operations are performed using the content provider via a content resolver.

The first step is to modify the MyDBHandler.java class so that it obtains a reference to a ContentResolver instance. This can be achieved in the constructor method of the class:

package com.example.database;

import com.example.database.provider.ContentProvider;

import android.content.ContentResolver;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteDatabase.CursorFactory;
import android.database.sqlite.SQLiteOpenHelper;

public class MyDBHandler extends SQLiteOpenHelper {

	private ContentResolver myCR;
	
	private static final int DATABASE_VERSION = 1;
	private static final String DATABASE_NAME = "productDB.db";
	public static final String TABLE_PRODUCTS = "products";
	
	public static final String COLUMN_ID = "_id";
	public static final String COLUMN_PRODUCTNAME = "productname";
	public static final String COLUMN_QUANTITY = "quantity";
	
	public MyDBHandler(Context context, String name, CursorFactory factory,
			int version) {
		super(context, DATABASE_NAME, factory, DATABASE_VERSION);
		myCR = context.getContentResolver();
	}
.
.
}

Next, the addProduct(), findProduct() and removeProduct() methods need to be re-written to use the content resolver and content provider for data management purposes:

public void addProduct(Product product) {

	ContentValues values = new ContentValues();
	values.put(COLUMN_PRODUCTNAME, product.getProductName());
	values.put(COLUMN_QUANTITY, product.getQuantity());
	 
	myCR.insert(ContentProvider.CONTENT_URI, values);
}

public Product findProduct(String productname) {
    	String[] projection = {COLUMN_ID,
    	   COLUMN_PRODUCTNAME, COLUMN_QUANTITY };
    	
    	String selection = "productname = \"" + productname + "\"";
    	
    	Cursor cursor = myCR.query(ContentProvider.CONTENT_URI, 
              projection, selection, null,
    	        null);
    	
	Product product = new Product();
		
	if (cursor.moveToFirst()) {
		cursor.moveToFirst();
		product.setID(Integer.parseInt(cursor.getString(0)));
		product.setProductName(cursor.getString(1));
			product.setQuantity(Integer.parseInt(cursor.getString(2)));
		cursor.close();
	} else {
		product = null;
	}
	return product;
}

public boolean deleteProduct(String productname) {
		
	boolean result = false;
    	
    	String selection = "productname = \"" + productname + "\"";
    	
    	int rowsDeleted = myCR.delete(ContentProvider.CONTENT_URI, 
                  selection, null);
		
    	if (rowsDeleted > 0)
    		result = true;
    	
    	return result;
}

With the database handler class updated to use a content resolver and content provider, the application is now ready to be tested. Compile and run the application and perform some operations to add, find and remove product entries. In terms of operation and functionality, the application should behave exactly as it did when directly accessing the database, except that it is now using the content provider.

With the content provider now implemented and declared in the manifest file, any other applications can potentially access that data (since no permissions were declared the default full access is in effect). The only information that the other applications need to know to gain access is the content URI and the names of the columns in the products table.

The goal of this chapter has been to provide a more detailed overview of the exact steps involved in implementing an Android content provider with a particular emphasis on the structure and implementation of the query, insert, delete and update methods of the content provider class. Practical use of the content resolver class to access data in the content provider was also covered, and the Database project created in the SQLite tutorial modified to make use of both a content provider and content resolver.

<google>BUY_KINDLE_FIRE</google>