Android 中利用 ContentResolver 查询数据库的四种方式

直接使用 ContentResolver 的 query 方法在主线程中查询数据（非异步查询）

由于是在主线程中执行查询操作，为了不阻塞主线程，尽量在数据量小的时候使用，而且要注意及时关闭 cursor

代码示例

Cursor cursor = getContentResolver().query(ContactsContract.Data.CONTENT_URI, null, null, null, null);
// 获得 cursor 中数据 ...
cursor.close();

利用 Activity 的 managedQuery 方法在主线程中查询数据（非异步查询）

相对于上一种方式，我们不需要处理 cusor 的关闭，但是同样只适合在数据量小的时候。

对于 Android 3.0 HONEYCOMB 以上的版本，Android 官方建议使用 LoaderManager 替代，这也是下一种查询方式。

代码示例：

Cursor cursor = managedQuery(ContactsContract.Data.CONTENT_URI, null, null, null, null);
// 获得 cursor 中数据 ...

利用 LoaderManager 机制中的 CursorLoader 异步查询数据库（异步查询）

Android3.0 以上可以直接使用，3.0 以下需要使用 support 库.

注：LoaderManager 开始一个 Loader 之后，如果没有调用 destroyLoader 方法，在 activity 的 onStart 时候会重新开始 Loader 中的任务

代码示例：

public class SampleListActivity extends ListActivity implements
    LoaderManager.LoaderCallbacks<Cursor> {

  private static final String[] PROJECTION = new String[] { "_id", "text_column" };

  // The loader's unique id. Loader ids are specific to the Activity or
  // Fragment in which they reside.
  private static final int LOADER_ID = 1;

  // The callbacks through which we will interact with the LoaderManager.
  private LoaderManager.LoaderCallbacks<Cursor> mCallbacks;

  // The adapter that binds our data to the ListView
  private SimpleCursorAdapter mAdapter;

  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    String[] dataColumns = { "text_column" };
    int[] viewIDs = { R.id.text_view };

    // Initialize the adapter. Note that we pass a 'null' Cursor as the
    // third argument. We will pass the adapter a Cursor only when the
    // data has finished loading for the first time (i.e. when the
    // LoaderManager delivers the data to onLoadFinished). Also note
    // that we have passed the '0' flag as the last argument. This
    // prevents the adapter from registering a ContentObserver for the
    // Cursor (the CursorLoader will do this for us!).
    mAdapter = new SimpleCursorAdapter(this, R.layout.list_item,
        null, dataColumns, viewIDs, 0);

    // Associate the (now empty) adapter with the ListView.
    setListAdapter(mAdapter);

    // The Activity (which implements the LoaderCallbacks<Cursor>
    // interface) is the callbacks object through which we will interact
    // with the LoaderManager. The LoaderManager uses this object to
    // instantiate the Loader and to notify the client when data is made
    // available/unavailable.
    mCallbacks = this;

    // Initialize the Loader with id '1' and callbacks 'mCallbacks'.
    // If the loader doesn't already exist, one is created. Otherwise,
    // the already created Loader is reused. In either case, the
    // LoaderManager will manage the Loader across the Activity/Fragment
    // lifecycle, will receive any new loads once they have completed,
    // and will report this new data back to the 'mCallbacks' object.
    LoaderManager lm = getLoaderManager();
    lm.initLoader(LOADER_ID, null, mCallbacks);
  }

  @Override
  public Loader<Cursor> onCreateLoader(int id, Bundle args) {
    // Create a new CursorLoader with the following query parameters.
    return new CursorLoader(SampleListActivity.this, CONTENT_URI,
        PROJECTION, null, null, null);
  }

  @Override
  public void onLoadFinished(Loader<Cursor> loader, Cursor cursor) {
    // A switch-case is useful when dealing with multiple Loaders/IDs
    switch (loader.getId()) {
      case LOADER_ID:
        // The asynchronous load is complete and the data
        // is now available for use. Only now can we associate
        // the queried Cursor with the SimpleCursorAdapter.
        mAdapter.swapCursor(cursor);
        break;
    }
    // The listview now displays the queried data.
  }

  @Override
  public void onLoaderReset(Loader<Cursor> loader) {
    // For whatever reason, the Loader's data is now unavailable.
    // Remove any references to the old data by replacing it with
    // a null Cursor.
    mAdapter.swapCursor(null);
  }
}

使用 Android 提供的异步查询框架 AsyncQueryHandler（异步查询）

AsyncQueryHandler 对比 CursorLoader 来说， 适合单次查询获得结果的情况。

代码示例：

private static final int TOKEN_QUERY_CONTACT = 1;

private MyAsyncQueryHandler mQueryHandler;

mQueryHandler = new MyAsyncQueryHandler(getContentResolver());
mQueryHandler.startQuery(TOKEN_QUERY_CONTACT, null, ContactsContract.Data.CONTENT_URI, null, null, null, null);

private final class MyAsyncQueryHandler extends AsyncQueryHandler {
    
    public MyAsyncQueryHandler (ContentResolver cr) {
        super(cr);
    }

    @Override
    protected void onQueryComplete(int token, Object cookie, Cursor cursor) {
        if(null == cursor) {
            return;
        }
        if(token == TOKEN_QUERY_CONTACT) {
            // 获得 cursor 中数据 ...
        }
    }
}

总结

1. 耗时非常短的操作可以直接用第一种方式

2. 对于只需要查询一次，不管数据量大小我觉得都最后使用 AsyncQueryHandler

3. 对于需要多次查询操作的，推荐使用 LoaderManager 机制的 CursorLoader

4. 数据库的异步更新、插入和删除推荐使用 AsyncQueryHandler

参考文章：

• Android利用ContentResolver查询的三种方式

• Understanding the LoaderManager (part 2)