单层数组嵌套示例：

{
 "_id": "someId",
 "arrayField": [
   {
     "fieldKey": "targetValue",
     "updateKey": "oldValue"
   },
   {
     "fieldKey": "otherValue",
     "updateKey": "value"
   }
 ]
} 

代码：

import com.mongodb.client.model.Filters;
import com.mongodb.client.model.Updates;
import org.bson.Document;
import org.bson.conversions.Bson;

// 构建匹配文档的查询条件
Bson match = Filters.eq("_id", "someId");

// 构建更新操作，目标是修改嵌套数组中特定对象的字段
Bson updateOperation = Updates.set("arrayField.$[elem].updateKey", "newValue");

// 指定数组过滤条件，用于匹配需要更新的数组元素
UpdateOptions options = new UpdateOptions().arrayFilters(Arrays.asList(Filters.eq("elem.fieldKey", "targetValue")));

// 执行更新操作
collection.updateOne(match, updateOperation, options);

两层数组嵌套示例：

{
  "_id": "someId",
  "arrayField": [
    {
      "fieldKey": "targetValue",
      "updateKey": [
          {
          "f1":"k1",
          "f2":"k2"
          },
          {
          "f1":"k3",
          "f2":"k4"
          }
      ]
    },
    {
      "fieldKey": "otherValue",
      "updateKey": [
          {
          "f1":"k1",
          "f2":"k2"
          },
          {
          "f1":"k3",
          "f2":"k4"
          }
      ]
    }
  ]
}

代码如下：

import com.mongodb.client.MongoClients;
import com.mongodb.client.MongoClient;
import com.mongodb.client.MongoDatabase;
import com.mongodb.client.MongoCollection;
import org.bson.Document;
import static com.mongodb.client.model.Filters.eq;
import static com.mongodb.client.model.Filters.and;
import static com.mongodb.client.model.Updates.set;
import com.mongodb.client.model.UpdateOptions;
import com.mongodb.client.result.UpdateResult;

public class MongoDBUpdateExample {
    public static void main(String[] args) {
        try (MongoClient mongoClient = MongoClients.create("mongodb://localhost:27017")) {
            MongoDatabase database = mongoClient.getDatabase("yourDatabaseName");
            MongoCollection<Document> collection = database.getCollection("yourCollectionName");

            // 构建查询条件，匹配f2等于k4的文档
            Document matchCondition = new Document("arrayField.updateKey.f2", "k4");
            
            // 构建更新操作，这里需要使用$操作符来精确指定路径
            Document updateOperation = new Document("$set", new Document("arrayField.$.updateKey.$[elem].f2", "new_value"));
            
            // 构建数组过滤器，以便只更新匹配的子文档
            UpdateOptions options = new UpdateOptions().arrayFilters(
                    java.util.Arrays.asList(new Document("elem.f2", "k4"))
            );
            
            UpdateResult result = collection.updateMany(matchCondition, updateOperation, options);
            
            System.out.println("Matched count: " + result.getMatchedCount());
            System.out.println("Modified count: " + result.getModifiedCount());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}