在android中使用Realm数据库框架

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详细参考说明在https://realm.io/docs/java/latest/#getting-started
Github地址在https://github.com/realm/realm-java
具体要使用该框架需要参考文档Getting Started中配置
Realm使用C++编写，不同于android中的sqlite

下面看一张结构图，

realm重要的对象，
Realm，RealmConfiguration，RealmObject，RealmList，RealmQuery，RealmResults这些概念想要了解可以去看文档。

1.在application中初始化

<span style="font-size:14px;">public class MyApplication extends Application {    @Override    public void onCreate() {        super.onCreate();        RealmConfiguration realmConfiguration = new RealmConfiguration.Builder(this).build();        Realm.setDefaultConfiguration(realmConfiguration);    }}</span>

2.在activity或者fragment中获取和关闭realm

<span style="font-size:14px;">public class MyActivity extends Activity {    private Realm realm;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        realm = Realm.getDefaultInstance();    }    @Override    protected void onDestroy() {        super.onDestroy();        realm.close();    }}</span>

<span style="font-size:14px;">public class MyFragment extends Fragment {    private Realm realm;    @Override    public void onStart() {        super.onStart();        realm = Realm.getDefaultInstance();    }    @Override    public void onStop() {        super.onStop();        realm.close();    }}</span>

3.使用realm来操作数据库，尽管这些操作很快，但是还是建议放在后台进行。可以在回调中更新ui

<span style="font-size:14px;">realm.executeTransactionAsync(new Realm.Transaction() {            @Override            public void execute(Realm bgRealm) {                User user = bgRealm.createObject(User.class);                user.setName("John");                user.setEmail("john@corporation.com");            }        }, new Realm.Transaction.OnSuccess() {            @Override            public void onSuccess() {                // Transaction was a success.            }        }, new Realm.Transaction.OnError() {            @Override            public void onError(Throwable error) {                // Transaction failed and was automatically canceled.            }        });</span>

这里要注意，你监听了并且在回调的方法中执行了ui操作。可能你执行ui操作之前activity或者fragment已经关闭

，所以我们要及时关闭realm任务，避免程序crash

<span style="font-size:14px;">@Overridepublic void onStop () {    if (transaction != null && !transaction.isCancelled()) {        transaction.cancel();    }}</span>

4.加密数据库

<span style="font-size:14px;">byte[] key = new byte[64];     new SecureRandom().nextBytes(key);    RealmConfiguration realmConfiguration = new RealmConfiguration.Builder(this)                .encryptionKey(key)                .build();        // Start with a clean slate every time     Realm.deleteRealm(realmConfiguration);        // Open the Realm with encryption enabled     realm = Realm.getInstance(realmConfiguration);</span>

5.一些基本的常用的realm数据库操作。

支持boolean, byte, short, ìnt, long, float, double, String, Date and byte[]；
其中byte, short, int, and long会自动映射成long

a,建立实体对象，继承RealmObject

RealmObject是自动更新的，比如你将一个数据读取出来后，又做了修改，修改会马上在数据中生效，不需要你再
将数据保存到数据库

<span style="font-size:14px;">public class User extends RealmObject {//realm不支持final, transient and volatile 修饰符@PrimaryKey  //定义主键private int id;@Required  //表示realm会检查该字段不允许为空    private String          name;    private int             age;        @Ignore   //表示realm会在操作数据库时忽略该字段，不会存放    private int             sessionId;    // Standard getters & setters generated by your IDE…    public String getName() { return name; }    public void   setName(String name) { this.name = name; }    public int    getAge() { return age; }    public void   setAge(int age) { this.age = age; }    public int    getSessionId() { return sessionId; }    public void   setSessionId(int sessionId) { this.sessionId = sessionId; }}</span>

当然也支持绑定一个reamlobject,来建立数据库中一对一，一对多，多对多的关系
比如一对一：

<span style="font-size:14px;">public class Email extends RealmObject {    private String address;    private boolean active;    // ... setters and getters left out}public class Contact extends RealmObject {    private String name;    private Email email;     // ... setters and getters left out}</span>

比如一对多，多对多，使用RealmList

<span style="font-size:14px;">public class Contact extends RealmObject {    public String name;    public RealmList<Email> emails;}public class Email extends RealmObject {    public String address;    public boolean active;}</span>

b.根据上面的介绍，建立好实体对象和完成reaml的初始化和获取后。就可以开始幸福的简单的数据库操作了

包含增删改查；
下面的操作都要放到executeTransactionAsync中
1.增加记录，

<span style="font-size:14px;">User user = realm.createObject(User.class);user.setName("John");user.setEmail("john@corporation.com");</span>

2.查询记录，

RealmResults<User> result2 = realm.where(User.class)  //以where表示条件的开头中间表示条件                                  .equalTo("name", "John")                                  .or()                                  .equalTo("name", "Peter")                                                                    .findAll();   //以findAll表示条件的结尾    可以不要条件，表示全部查找。也可以将多个连续查询合并，示例如下    RealmResults<User> r2 = realm.where(User.class)                             .equalTo("dogs.name", "Fluffy")                             .findAll()                                                          .where()                             .equalTo("dogs.color", "Brown")                             .findAll();                                                          .where()                             .equalTo("dogs.color", "Yellow")                             .findAll();

一些常用的查找条件，between(), greaterThan(), lessThan(), greaterThanOrEqualTo() & lessThanOrEqualTo()equalTo() & notEqualTo()contains(), beginsWith() & endsWith()isNull() & isNotNull()isEmpty() & isNotEmpty()可以查完后排序RealmResults<User> result = realm.where(User.class).findAll();result = result.sort("age"); // Sort ascendingresult = result.sort("age", Sort.DESCENDING);可以查完后进行求和，求最大最小，求平均RealmResults<User> results = realm.where(User.class).findAll();long   sum     = results.sum("age").longValue();long   min     = results.min("age").longValue();long   max     = results.max("age").longValue();double average = results.average("age");long   matches = results.size();

3.删除记录，搜索出需要删除的记录后调用

final RealmResults<User> results = getUsers();results.deleteFromRealm(0);     results.deleteFirstFromRealm();    results.deleteLastFromRealm();        results.deleteAllFromRealm();

4.修改记录，直接查完后调用set方法就可以了。

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