下面是一种获得所需结果的方法。

Document projectn = new Document("$project", 
                        new Document("_id", 0L)
                        .append("file_name", 1L)
                        .append("vehicle_id", 1L)
                        .append("errors", 
                            new Document("$map", 
                                new Document("input", "$errors")
                                .append("in", 
                                    new Document("priority", "$$this.priority")
                                    .append("category", "$$this.category")
                                )
                            )
                        )
                    );

List<Document> pipeline = Arrays.asList(projectn);
List<Document> results = mongoOps.getCollection("collection_name")
                                 .aggregate(pipeline)
                                 .into(new ArrayList<>());

注意，这里使用了MongoDB Java Driver API查询语法，Document是org.bson.Document。本地查询到Java Driver的转换使用了$map聚合数组运算符（看起来这是（可能是唯一的）方法）。
对于MongoTemplate.aggregate，代码为：

Aggregation agg = newAggregation(
            project() 
            .and( 
                VariableOperators.Map.itemsOf("errors")
                    .as("e")
                    .andApply(ctx -> new Document("priority", "$$e.priority").append("category", "$$e.category") ))
            .as("errors")
            .andExclude("_id")
            .andInclude("file_name", "vehicle_id")
    );

    AggregationResults<Document> results = mongoOps.aggregate(agg, "collection_name", Document.class);

替代方法：

如果你的查询只是关于 projection 的，你可以使用MongoTemplate#find方法来查询，这样构造和理解起来要简单得多：

db.collection.find(
    {}, // your query filter
    { _id: 0, 'errors.category': 1, 'errors.priority': 1, file_name: 1, vehicle_id: 1 }
)

它的MongoTemplate版本：

Query query = new Query();
query.fields()
     .include("errors.priority", "errors.category", "file_name", "vehicle_id")
     .exclude("_id");
List<Document> results = mongoOps.find(query, Document.class, "collection_name");

您需要使用nested()函数，如下所示：

AggregationOperation project = Aggregation.project("file_name", "vehicle_id").
    and("errors").nested(Fields.fields("priority","category"))

Aggregation aggregation = Aggregation.newAggregation(project);

List<String> projectFieldSet = Arrays.asList("errors.category","errors.priority");
List<Field> fieldList = projectFields.stream().map(f->{return Fields.field(f,f);}).collect(Collectors.toList());        
ProjectionOperation projectAggOp = Aggregation.project(Fields.from(fieldList.toArray(new Field[fieldList.size()])));