Question

我需要从这个Blob读取字节。我正在尝试以下但是我得到了这个例外： oracle.sql.BLOB无法强制转换为[B

(defn select-test2[]
  (clojure.contrib.sql/with-connection db
    (with-query-results res ["SELECT my_blob from some_table"] (doall res))))

(defn obj [byte-buffer]
  (if-not (nil? byte-buffer)
    (with-open [object-in (ObjectInputStream.
                            (ByteArrayInputStream. byte-buffer))]
      (.readObject object-in))))

(obj (:my_blob (first (select-test2))))

Answer 1

(ns test-jdbc
  (:use clojure.contrib.sql))

; read clob with BufferedReader
(defn clob-to-string [clob]
  (with-open [rdr (java.io.BufferedReader. (.getCharacterStream clob))]
    (apply str (line-seq rdr))))

; read first CLOB 
(defn get-meta-by-id [db id]
  "read META_COL from MY_TABLE by given id"
  (with-connection db
    (transaction ;; need to be inside a transaction
      (with-query-results rs 
        ["select META_COL from MY_TABLE where ID = ? " id]
        (clob-to-string (:meta-col (first rs))) ))))

Answer 2

[B是字节数组的“类”：

user=> (type (byte-array 0))
[B

因此，代码中有一个地方需要一个字节数组，但是它被赋予oracle.sql.Blob个实例。我敢打赌:my_blob给你一个Blob;当您将byte-buffer（Blob）传递给ByteArrayInputStream构造函数时，您将获得异常。

查找oracle.sql.Blob的javadoc，了解如何从中提取字节数组或输入流。

Answer 3

(defn blob-to-byte [blob]
    (let [ary (byte-array (.length blob))
          is (.getBinaryStream blob)]
    (.read is ary)
    (.close is)
    ary))

(j/query db/real-db ["select blob from table"]
         {:row-fn #(->> % :blob blob-to-byte)}))

Clojure从数据库中读取Blob

3 个答案: