Exits API
如果文檔存在則返回true、如果不存在則返回false
```
GetRequest getRequest = new GetRequest(
"posts",
"1");
getRequest.fetchSourceContext(new FetchSourceContext(false)); //Disable fetching_source
getRequest.storedFields("_none_");//禁用獲取存儲的字段
```
同步執行
```
boolean exists = client.exists(getRequest, RequestOptions.DEFAULT);
```
異步執行
~~~
client.existsAsync(getRequest, RequestOptions.DEFAULT, listener);
~~~
```
ActionListener<Boolean> listener = new ActionListener<Boolean>() {
@Override
public void onResponse(Boolean exists) {
}
@Override
public void onFailure(Exception e) {
}
};
```
