LevelDB Java Api

LevelDB是K-V数据库。

LevelDB在存储数据时，是根据记录的key值有序存储的，就是说相邻的key值在存储文件中是依次顺序存储的，而应用可以自定义key大小比较函数，LevleDb会按照用户定义的比较函数依序存储这些记录。

例如：int类型的键，levelDB默认按照自然顺序(升序)进行有序存储，其他类型可以自定义比较函数。

注：可以利用其有序存储这一特性进行范围查找。

LevelDB API的使用

1.导包命令：

import org.iq80.leveldb.*;
import static org.fusesource.leveldbjni.JniDBFactory.*;
import java.io.*;

2.关闭开启数据库

Options options = new Options();
options.createIfMissing(true);
DB db = factory.open(new File("example"), options);
try {
  // Use the db in here....
} finally {
  // Make sure you close the db to shutdown the 
  // database and avoid resource leaks.
  db.close();
}

3.增、查、删数据

db.put(bytes("Tampa"), bytes("rocks"));
String value = asString(db.get(bytes("Tampa")));
db.delete(bytes("Tampa"));

4.批量操作，Performing Batch/Bulk/Atomic Updates.

WriteBatch batch = db.createWriteBatch();
try {
  batch.delete(bytes("Denver"));
  batch.put(bytes("Tampa"), bytes("green"));
  batch.put(bytes("London"), bytes("red"));

  db.write(batch);
} finally {
  // Make sure you close the batch to avoid resource leaks.
  batch.close();
}

5. 遍历 Iterating key/values.

DBIterator iterator = db.iterator();
try {
  for(iterator.seekToFirst(); iterator.hasNext(); iterator.next()) {
    String key = asString(iterator.peekNext().getKey());
    String value = asString(iterator.peekNext().getValue());
    System.out.println(key+" = "+value);
  }
} finally {
  // Make sure you close the iterator to avoid resource leaks.
  iterator.close();
}

6. 快照 Working against a Snapshot view of the Database.

ReadOptions ro = new ReadOptions();
ro.snapshot(db.getSnapshot());
try {
  
  // All read operations will now use the same 
  // consistent view of the data.
  ... = db.iterator(ro);
  ... = db.get(bytes("Tampa"), ro);

} finally {
  // Make sure you close the snapshot to avoid resource leaks.
  ro.snapshot().close();
}

7. 自定义比较器 Using a custom Comparator.

DBComparator comparator = new DBComparator(){
    public int compare(byte[] key1, byte[] key2) {
        return new String(key1).compareTo(new String(key2));
    }
    public String name() {
        return "simple";
    }
    public byte[] findShortestSeparator(byte[] start, byte[] limit) {
        return start;
    }
    public byte[] findShortSuccessor(byte[] key) {
        return key;
    }
};
Options options = new Options();
options.comparator(comparator);
DB db = factory.open(new File("example"), options);

8. 禁用压缩 Disabling Compression

Options options = new Options();
options.compressionType(CompressionType.NONE);
DB db = factory.open(new File("example"), options);

9.配置缓存 Configuring the Cache

Options options = new Options();
options.cacheSize(100 * 1048576); // 100MB cache
DB db = factory.open(new File("example"), options);

9.获得近似尺寸 Getting approximate sizes.

long[] sizes = db.getApproximateSizes(new Range(bytes("a"), bytes("k")), new Range(bytes("k"), bytes("z")));
System.out.println("Size: "+sizes[0]+", "+sizes[1]);

10.获取数据库状态 Getting database status.

String stats = db.getProperty("leveldb.stats");
System.out.println(stats);

11.获取日志信息 Getting informational log messages.

Logger logger = new Logger() {
  public void log(String message) {
    System.out.println(message);
  }
};
Options options = new Options();
options.logger(logger);
DB db = factory.open(new File("example"), options);

12.销毁数据库 Destroying a database.

Options options = new Options();
factory.destroy(new File("example"), options);

13.修复数据库 Repairing a database.

Options options = new Options();
factory.repair(new File("example"), options);

14使用内存池可以提高本机内存分配的效率： Using a memory pool to make native memory allocations more efficient:

JniDBFactory.pushMemoryPool(1024 * 512);
try {
    // .. work with the DB in here, 
} finally {
    JniDBFactory.popMemoryPool();
}

 参考地址：

https://github.com/fusesource/leveldbjni