Berkely DB Java Edition学习笔记

Berkely DB对于高并发、要求速度快的应用来说是个不错的选择，mysql就是用BDB实现的(mysql的后台) ，mysql快，BDB比mysql还要快N倍。BDB是一种嵌入式的、非关系数据库，它与其他的关系数据库RMDBS不同,它没有提供SQL,而是提供了自己的访问接口。作为一种嵌入式的数据库，它是进程内模式的，也就是说它和应用程序在同一内存空间运行，所以速度要高很多，与嵌入式的数据库如Derby、HSQLDB(都是RMDBS的)相比，它效率更高，使用方法也有很大的不同。现在BDB以被Oracle收购。Berkely DB提供的文档Getting Started with　Berkeley DB Java Edition可以说是短小精悍（１１３页），入门相当不错。下面Get Start吧：
Environment：
首先要接触的就是Environment了，使用它来open　database以及做一管理方面的事情.
创建Environment，还需要Environment的一些配置信息EnvironmentConfig。
下面是创建的过程：

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EnvironmentConfig envConfig = new EnvironmentConfig();  
envConfig.setAllowCreate(true);  
myDbEnvironment = new Environment(new File("/export/dbEnv"),  
envConfig); 

EnvironmentConfig envConfig = new EnvironmentConfig();
envConfig.setAllowCreate(true);
myDbEnvironment = new Environment(new File("/export/dbEnv"),
envConfig);

其中EnvironmentConfig提供了许多配置参数，常用的有：
envConfig.setAllowCreate()//如果不存在的env的话，是否要创建一个新的
envConfig.setReadOnly()//是否为只读的
envConfig.setTransactional()//是否使用事务
参数都是boolean类型的
除了EnvironmentConfig外，还有EnvironmentMutableConfig，他实际是EnvironmentConfig的父类，使用他来配置在创建完Environment之后可以改变
的属性：
setCachePercent()//设置cache的大小占JVM　memory的百分比
setCacheSize()//设置cache的大小
setTxnNoSync()//事务提交是否将改变的记录写入磁盘
setTxnWriteNoSync()//事务提交是否将log写入磁盘
下面看一下使用EnvironmentMutableConfig的方法:

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Environment myEnv = new Environment(new File("/export/dbEnv"), null);  
EnvironmentMutableConfig envMutableConfig =  
new EnvironmentMutableConfig();  
envMutableConfig.setTxnNoSync(true);  
myEnv.setMutableConfig(envMutableConfig); 

Environment myEnv = new Environment(new File("/export/dbEnv"), null);
EnvironmentMutableConfig envMutableConfig =
new EnvironmentMutableConfig();
envMutableConfig.setTxnNoSync(true);
myEnv.setMutableConfig(envMutableConfig);

Environment通过close来关闭，释放资源
下面看看Environment在管理方面的一些方法：
可以通过Environment获得EnvironmentStats，他提供了Environment一些状态信息，
例如使用

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long cacheMisses = myEnv.getStats(null).getNCacheMiss(); 

long cacheMisses = myEnv.getStats(null).getNCacheMiss();

我们可以获得cache未命中的次数，据此来调整cache的大小
可以同过Environment.getDatabaseNames()来获得Environment的数据库的名字：

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List myDbNames = myDbEnv.getDatabaseNames();  
for(int i=0; i < myDbNames.size(); i++) {  
System.out.println("Database Name: " + (String)myDbNames.get(i));  
} 

List myDbNames = myDbEnv.getDatabaseNames();
for(int i=0; i < myDbNames.size(); i++) {
System.out.println("Database Name: " + (String)myDbNames.get(i));
}

可以通过Environment.removeDatabase()来删除一个数据库：

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String dbName = myDB.getDatabaseName();  
myDB.close();  
myDBEnv.removeDatabase(null,dbName); 

String dbName = myDB.getDatabaseName();
myDB.close();
myDBEnv.removeDatabase(null,dbName);

可以使用Environment.renameDatabase()来重新命名一个数据库：

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String dbName = myDB.getDatabaseName();  
String dbNewName = new String(dbName + ".new", "UTF-8");  
myDB.close();  
myDBEnv.renameDatabase(null,dbName,dbNewName); 

String dbName = myDB.getDatabaseName();
String dbNewName = new String(dbName + ".new", "UTF-8");
myDB.close();
myDBEnv.renameDatabase(null,dbName,dbNewName);

可以使用Environment.truncateDatabase()来删除数据库中的所有记录：

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myEnv.truncate(null, // txn handle  
myDatabase.getDatabaseName(), // database name  
true//whether to return the count of deleted records  
); 

myEnv.truncate(null, // txn handle
myDatabase.getDatabaseName(), // database name
true//whether to return the count of deleted records
);

第三个参数是否返回删除的记录数，性能有很大不同。false的话会很快
Database:
最重要的一些操作大多都在Database里了，和Environment一样，它也有许多
配置的选项DatabaseConfig,我们先看看选项：
DatabaseConfig.setAllowCreate()//不能存在的话是open操作否创建新的
DatabaseConfig.setBtreeComparator()//设置Btree的比较器
DatabaseConfig.setDuplicateComparator()//设置判断重复的比较器
DatabaseConfig.setSortedDuplicates()//是否允许重复的记录
DatabaseConfig.setExclusiveCreate()//设为true,如果当前数据库已存在，则open失败，也就是说open操作会导致一个新的数据库被创建，默认为false
DatabaseConfig.setReadOnly()//是否是只读的
DatabaseConfig.setTransactional()//是否使用事务
下面我们看看Database的使用流程：

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EnvironmentConfig envConfig = new EnvironmentConfig();  
envConfig.setAllowCreate(true);  
myDbEnvironment = new Environment(new File("/export/dbEnv"), envConfig);  
DatabaseConfig dbConfig = new DatabaseConfig();  
dbConfig.setAllowCreate(true);  
myDatabase = myDbEnvironment.openDatabase(null,  
"sampleDatabase",  
dbConfig); 

EnvironmentConfig envConfig = new EnvironmentConfig();
envConfig.setAllowCreate(true);
myDbEnvironment = new Environment(new File("/export/dbEnv"), envConfig);
DatabaseConfig dbConfig = new DatabaseConfig();
dbConfig.setAllowCreate(true);
myDatabase = myDbEnvironment.openDatabase(null,
"sampleDatabase",
dbConfig);

我们通过Environment的openDatabase来创建Database对象。使用完了Database使用
close方法来关闭数据库释放资源。
Database Records
Database Record是保存在数据库的内容，包含Key和value两部分，他们都被封装成
DatabaseEntry，DatabaseEntry只能存放字节数组，所以只要能把Key和Value是什么
类型的，只要能转化成字节数组就可以被DatabaseEntry封装。基本类型JE都有对应的Binding,复杂的类型可以使用序列化和自定义binding来实现。
下那面我们看看一个使用方法：

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String aKey = "key";  
String aData = "data";  
try {  
DatabaseEntry theKey = new DatabaseEntry(aKey.getBytes("UTF-8"));  
DatabaseEntry theData = new DatabaseEntry(aData.getBytes("UTF-8"));  
} catch (Exception e) {  
} 

String aKey = "key";
String aData = "data";
try {
DatabaseEntry theKey = new DatabaseEntry(aKey.getBytes("UTF-8"));
DatabaseEntry theData = new DatabaseEntry(aData.getBytes("UTF-8"));
} catch (Exception e) {
}

我们不应该依赖机器默认的编码，通常要指定特定的编码方法getBytes("UTF-8");
我们先看看怎么从数据库中读写记录：
通过Database.put()和Database.get()我们可以从数据库中读写记录
put:

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String aKey = "myFirstKey";  
String aData = "myFirstData";  
try {  
DatabaseEntry theKey = new DatabaseEntry(aKey.getBytes("UTF-8"));  
DatabaseEntry theData = new DatabaseEntry(aData.getBytes("UTF-8"));  
myDatabase.put(null, theKey, theData);  
} catch (Exception e) {  
} 

String aKey = "myFirstKey";
String aData = "myFirstData";
try {
DatabaseEntry theKey = new DatabaseEntry(aKey.getBytes("UTF-8"));
DatabaseEntry theData = new DatabaseEntry(aData.getBytes("UTF-8"));
myDatabase.put(null, theKey, theData);
} catch (Exception e) {
}

get:

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String aKey = "myFirstKey";  
try {  
DatabaseEntry theKey = new DatabaseEntry(aKey.getBytes("UTF-8"));  
DatabaseEntry theData = new DatabaseEntry();  
 
if (myDatabase.get(null, theKey, theData, LockMode.DEFAULT) ==  
OperationStatus.SUCCESS) {  
 
byte[] retData = theData.getData();  
String foundData = new String(retData, "UTF-8");  
System.out.println("For key: '" + aKey + "' found data: '" +  
foundData + "'.");  
} else {  
  System.out.println("No record found for key '" + aKey + "'.");  
}  
} catch (Exception e) {  
} 

String aKey = "myFirstKey";
try {
DatabaseEntry theKey = new DatabaseEntry(aKey.getBytes("UTF-8"));
DatabaseEntry theData = new DatabaseEntry();

if (myDatabase.get(null, theKey, theData, LockMode.DEFAULT) ==
OperationStatus.SUCCESS) {

byte[] retData = theData.getData();
String foundData = new String(retData, "UTF-8");
System.out.println("For key: '" + aKey + "' found data: '" +
foundData + "'.");
} else {
  System.out.println("No record found for key '" + aKey + "'.");
}
} catch (Exception e) {
}

删除操作:

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String aKey = "myFirstKey";  
DatabaseEntry theKey = new DatabaseEntry(aKey.getBytes("UTF-8"));  
myDatabase.delete(null, theKey); 

String aKey = "myFirstKey";
DatabaseEntry theKey = new DatabaseEntry(aKey.getBytes("UTF-8"));
myDatabase.delete(null, theKey);

使用BIND APIs来操作基本类型：
我们可以使用JE提供的Bind Apis来操作数字类型和字符串类型：
以Long为例：
存储数据使用Bind Apis一般步骤如下：
1、通过EntryBinding binding =TupleBinding.getPrimitiveBinding(Long.class);
2、通过EntryBinding 把数据放到DatabaseEntry中：
myBinding.objectToEntry(data, dataEntry);
获取数据使用Bind Apis一般步骤如下：
1、通过EntryBinding binding =TupleBinding.getPrimitiveBinding(Long.class);
2、通过EntryBinding将Entry转换成Object Long theLong = (Long) myBinding.entryToObject(theData);
下面代码以测试的形式演示了整个过程：

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package edu.jlu.fuliang;  
 
import java.io.File;  
 
import com.sleepycat.bind.EntryBinding;  
import com.sleepycat.bind.tuple.TupleBinding;  
import com.sleepycat.je.Database;  
import com.sleepycat.je.DatabaseConfig;  
import com.sleepycat.je.DatabaseEntry;  
import com.sleepycat.je.Environment;  
import com.sleepycat.je.EnvironmentConfig;  
import com.sleepycat.je.LockMode;  
 
import junit.framework.TestCase;  
 
public class PrimitiveBindingTest extends TestCase{  
    private Environment env;  
    private Database db;  
    private String key = "akey";  
    private Long data = 1234556633L;  
      
    public void setUp()throws Exception{  
        EnvironmentConfig envConfig = new EnvironmentConfig();  
        envConfig.setAllowCreate(true);  
        env = new Environment(new File("etc/dbEnv"),envConfig);  
        DatabaseConfig dbConfig = new DatabaseConfig();  
        dbConfig.setAllowCreate(true);  
        db = env.openDatabase(null, "myDB", dbConfig);  
        DatabaseEntry keyEntry = new DatabaseEntry(key.getBytes("UTF-8"));  
        DatabaseEntry dataEntry = new DatabaseEntry();  
          
        EntryBinding myBinding = TupleBinding.getPrimitiveBinding(Long.class);  
        myBinding.objectToEntry(data, dataEntry);  
        db.put(null, keyEntry, dataEntry);  
    }  
      
    public void testGet()throws Exception{  
        DatabaseEntry keyEntry = new DatabaseEntry(key.getBytes("UTF-8"));  
        DatabaseEntry dataEntry = new DatabaseEntry();  
        EntryBinding binding = TupleBinding.getPrimitiveBinding(Long.class);  
        db.get(null, keyEntry, dataEntry, LockMode.DEFAULT);  
        Long l = (Long)binding.entryToObject(dataEntry);  
        assertEquals(l,data);  
    }  
    public void tearDown()throws Exception{  
        db.close();  
        env.truncateDatabase(null, "myDB",false);  
        env.close();  
    }  
      
} 

package edu.jlu.fuliang;

import java.io.File;

import com.sleepycat.bind.EntryBinding;
import com.sleepycat.bind.tuple.TupleBinding;
import com.sleepycat.je.Database;
import com.sleepycat.je.DatabaseConfig;
import com.sleepycat.je.DatabaseEntry;
import com.sleepycat.je.Environment;
import com.sleepycat.je.EnvironmentConfig;
import com.sleepycat.je.LockMode;

import junit.framework.TestCase;

public class PrimitiveBindingTest extends TestCase{
 private Environment env;
 private Database db;
 private String key = "akey";
 private Long data = 1234556633L;
 
 public void setUp()throws Exception{
  EnvironmentConfig envConfig = new EnvironmentConfig();
  envConfig.setAllowCreate(true);
  env = new Environment(new File("etc/dbEnv"),envConfig);
  DatabaseConfig dbConfig = new DatabaseConfig();
  dbConfig.setAllowCreate(true);
  db = env.openDatabase(null, "myDB", dbConfig);
  DatabaseEntry keyEntry = new DatabaseEntry(key.getBytes("UTF-8"));
  DatabaseEntry dataEntry = new DatabaseEntry();
  
  EntryBinding myBinding = TupleBinding.getPrimitiveBinding(Long.class);
     myBinding.objectToEntry(data, dataEntry);
     db.put(null, keyEntry, dataEntry);
 }
 
 public void testGet()throws Exception{
  DatabaseEntry keyEntry = new DatabaseEntry(key.getBytes("UTF-8"));
  DatabaseEntry dataEntry = new DatabaseEntry();
  EntryBinding binding = TupleBinding.getPrimitiveBinding(Long.class);
  db.get(null, keyEntry, dataEntry, LockMode.DEFAULT);
  Long l = (Long)binding.entryToObject(dataEntry);
  assertEquals(l,data);
 }
 public void tearDown()throws Exception{
  db.close();
  env.truncateDatabase(null, "myDB",false);
  env.close();
 }
 
}

序列化复杂的类型
步骤如下：
1、要存储的对象的类需要实现java.io.Serializable
2、打开两个数据库，一个存放数据，另一个存放类的信息
3、实例化com.sleepycat.bind.serial.StoredClassCatalog对象
4、创建uses com.sleepycat.bind.serial.SerialBinding对象
5、使用SerialBinding把对象放到DatabaseEntry中
下面是使用一个能够完整描述这个过程的例子来说明这个过程：

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package edu.jlu.fuliang;  
 
import java.io.File;  
 
import com.sleepycat.bind.EntryBinding;  
import com.sleepycat.bind.serial.SerialBinding;  
import com.sleepycat.bind.serial.StoredClassCatalog;  
import com.sleepycat.je.Database;  
import com.sleepycat.je.DatabaseConfig;  
import com.sleepycat.je.DatabaseEntry;  
import com.sleepycat.je.DatabaseException;  
import com.sleepycat.je.Environment;  
import com.sleepycat.je.EnvironmentConfig;  
import com.sleepycat.je.LockMode;  
 
import junit.framework.TestCase;  
 
public class SerializableTypeTest extends TestCase{  
    private Person person;  
    private Environment env;  
    private Database db,classDB;  
    private StoredClassCatalog classCatalog;  
      
    public void setUp()throws Exception{  
        person = new Person();  
        person.setAge(12);  
        person.setName("zhansan");  
        person.setSex('m');  
          
        EnvironmentConfig envConfig = new EnvironmentConfig();  
        envConfig.setAllowCreate(true);  
        env = new Environment(new File("etc/dbEnv"),envConfig);  
        DatabaseConfig dbConfig = new DatabaseConfig();  
        dbConfig.setAllowCreate(true);  
        db = env.openDatabase(null, "myDB", dbConfig);  
        classDB = env.openDatabase(null, "classDB", dbConfig);  
        classCatalog = new StoredClassCatalog(classDB);  
        EntryBinding dataBinding = new SerialBinding(classCatalog,Person.class);  
              
        DatabaseEntry keyEntry = new DatabaseEntry(person.getName().getBytes("UTF-8"));  
        DatabaseEntry dataEntry = new DatabaseEntry();  
        dataBinding.objectToEntry(person, dataEntry);  
        db.put(null, keyEntry, dataEntry);  
    }  
      
    public void testGet()throws Exception{  
        EntryBinding dataBinding = new SerialBinding(classCatalog,Person.class);  
        
        DatabaseEntry keyEntry = new DatabaseEntry(person.getName().getBytes("UTF-8"));  
        DatabaseEntry dataEntry = new DatabaseEntry();  
        db.get(null, keyEntry, dataEntry, LockMode.DEFAULT);  
        Person p = (Person)dataBinding.entryToObject(dataEntry);  
        assertEquals(p.getName(),person.getName());  
        assertEquals(p.getAge(),person.getAge());  
        assertEquals(p.getSex(), person.getSex());  
    }  
      
    public void tearDown()throws Exception{  
        db.close();  
        classDB.close();  
        env.truncateDatabase(null, "myDB", false);  
        env.truncateDatabase(null, "classDB", false);  
        env.close();  
    }  
} 

package edu.jlu.fuliang;

import java.io.File;

import com.sleepycat.bind.EntryBinding;
import com.sleepycat.bind.serial.SerialBinding;
import com.sleepycat.bind.serial.StoredClassCatalog;
import com.sleepycat.je.Database;
import com.sleepycat.je.DatabaseConfig;
import com.sleepycat.je.DatabaseEntry;
import com.sleepycat.je.DatabaseException;
import com.sleepycat.je.Environment;
import com.sleepycat.je.EnvironmentConfig;
import com.sleepycat.je.LockMode;

import junit.framework.TestCase;

public class SerializableTypeTest extends TestCase{
 private Person person;
 private Environment env;
 private Database db,classDB;
 private StoredClassCatalog classCatalog;
 
 public void setUp()throws Exception{
  person = new Person();
  person.setAge(12);
  person.setName("zhansan");
  person.setSex('m');
    
  EnvironmentConfig envConfig = new EnvironmentConfig();
  envConfig.setAllowCreate(true);
  env = new Environment(new File("etc/dbEnv"),envConfig);
  DatabaseConfig dbConfig = new DatabaseConfig();
  dbConfig.setAllowCreate(true);
  db = env.openDatabase(null, "myDB", dbConfig);
  classDB = env.openDatabase(null, "classDB", dbConfig);
  classCatalog = new StoredClassCatalog(classDB);
  EntryBinding dataBinding = new SerialBinding(classCatalog,Person.class);
   
  DatabaseEntry keyEntry = new DatabaseEntry(person.getName().getBytes("UTF-8"));
  DatabaseEntry dataEntry = new DatabaseEntry();
  dataBinding.objectToEntry(person, dataEntry);
  db.put(null, keyEntry, dataEntry);
 }
 
 public void testGet()throws Exception{
  EntryBinding dataBinding = new SerialBinding(classCatalog,Person.class);
  
  DatabaseEntry keyEntry = new DatabaseEntry(person.getName().getBytes("UTF-8"));
     DatabaseEntry dataEntry = new DatabaseEntry();
     db.get(null, keyEntry, dataEntry, LockMode.DEFAULT);
     Person p = (Person)dataBinding.entryToObject(dataEntry);
     assertEquals(p.getName(),person.getName());
     assertEquals(p.getAge(),person.getAge());
     assertEquals(p.getSex(), person.getSex());
 }
 
 public void tearDown()throws Exception{
  db.close();
  classDB.close();
  env.truncateDatabase(null, "myDB", false);
  env.truncateDatabase(null, "classDB", false);
  env.close();
 }
}

要存储的对象对应的类

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package edu.jlu.fuliang;  
 
import java.io.Serializable;  
 
public class Person implements Serializable{  
    private String name;  
    private int age;  
    private char sex;  
 
    public int getAge() {  
        return age;  
    }  
 
    public void setAge(int age) {  
        this.age = age;  
    }  
 
    public String getName() {  
        return name;  
    }  
 
    public void setName(String name) {  
        this.name = name;  
    }  
 
    public char getSex() {  
        return sex;  
    }  
 
    public void setSex(char sex) {  
        this.sex = sex;  
    }  
} 

package edu.jlu.fuliang;

import java.io.Serializable;

public class Person implements Serializable{
 private String name;
 private int age;
 private char sex;

 public int getAge() {
  return age;
 }

 public void setAge(int age) {
  this.age = age;
 }

 public String getName() {
  return name;
 }

 public void setName(String name) {
  this.name = name;
 }

 public char getSex() {
  return sex;
 }

 public void setSex(char sex) {
  this.sex = sex;
 }
}

自定义元组绑定：
存储复杂对象自定义元组绑定的步骤：
1、创建要存储的对象，这个对象的类没有必要实现Serializable接口：
2、扩展com.sleepycat.bind.tuple.TupleBinding来实现自定义的Binging
3、创建2步欻关键的自定义binding对象
4、将创建的对象是用自定义个binding放到DatabaseEntry中
5、使用put方法存入数据库
下面的例子说明了这个过程：
自定义Binging：

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package edu.jlu.fuliang;  
 
import com.sleepycat.bind.tuple.TupleBinding;  
import com.sleepycat.bind.tuple.TupleInput;  
import com.sleepycat.bind.tuple.TupleOutput;  
 
public class PersonTupleBinding extends TupleBinding{  
 
    @Override 
    public Object entryToObject(TupleInput ti) {  
        Person person = new Person();  
        person.setName(ti.readString());  
        person.setAge(ti.readInt());  
        person.setSex(ti.readChar());  
        return person;  
    }  
 
    @Override 
    public void objectToEntry(Object obj, TupleOutput output) {  
        Person person = (Person)obj;  
        output.writeString(person.getName());  
        output.writeInt(person.getAge());  
        output.writeChar(person.getSex());  
    }  
} 

package edu.jlu.fuliang;

import com.sleepycat.bind.tuple.TupleBinding;
import com.sleepycat.bind.tuple.TupleInput;
import com.sleepycat.bind.tuple.TupleOutput;

public class PersonTupleBinding extends TupleBinding{

 @Override
 public Object entryToObject(TupleInput ti) {
  Person person = new Person();
  person.setName(ti.readString());
  person.setAge(ti.readInt());
  person.setSex(ti.readChar());
  return person;
 }

 @Override
 public void objectToEntry(Object obj, TupleOutput output) {
  Person person = (Person)obj;
  output.writeString(person.getName());
  output.writeInt(person.getAge());
  output.writeChar(person.getSex());
 }
}

put/get的使用过程：

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package edu.jlu.fuliang;  
 
import java.io.File;  
import com.sleepycat.je.Database;  
import com.sleepycat.je.DatabaseConfig;  
import com.sleepycat.je.DatabaseEntry;  
import com.sleepycat.je.DatabaseException;  
import com.sleepycat.je.Environment;  
import com.sleepycat.je.EnvironmentConfig;  
import com.sleepycat.je.LockMode;  
 
import junit.framework.TestCase;  
 
public class CustomTupleBindingTest extends TestCase{  
    private Person person;  
    private Environment env;  
    private Database db;  
      
    public void setUp()throws Exception{  
        person = new Person();  
        person.setAge(12);  
        person.setName("zhansan");  
        person.setSex('m');  
          
          
        EnvironmentConfig envConfig = new EnvironmentConfig();  
        envConfig.setAllowCreate(true);  
        env = new Environment(new File("etc/dbEnv"),envConfig);  
        DatabaseConfig dbConfig = new DatabaseConfig();  
        dbConfig.setAllowCreate(true);  
        db = env.openDatabase(null, "myDB", dbConfig);  
        PersonTupleBinding binding = new PersonTupleBinding();  
        DatabaseEntry keyEntry = new DatabaseEntry(person.getName().getBytes("UTF-8"));  
        DatabaseEntry dataEntry = new DatabaseEntry();  
        binding.objectToEntry(person, dataEntry);  
        db.put(null, keyEntry, dataEntry);  
          
    }  
      
    public void testGet()throws Exception{  
        PersonTupleBinding binding = new PersonTupleBinding();  
          
        DatabaseEntry keyEntry = new DatabaseEntry(person.getName().getBytes("UTF-8"));  
        DatabaseEntry dataEntry = new DatabaseEntry();  
        db.get(null, keyEntry, dataEntry, LockMode.DEFAULT);  
        Person p = (Person)binding.entryToObject(dataEntry);  
        assertEquals(p.getName(),person.getName());  
        assertEquals(p.getAge(),person.getAge());  
        assertEquals(p.getSex(), person.getSex());  
    }  
    public void tearDown()throws Exception{  
        db.close();  
        env.truncateDatabase(null, "myDB", false);  
        env.close();  
    }  
}