delay用法 c3p0 线程池 thread pool 数据源 datasource

00 [clojure delay用法].md

A Delay ensures that some code is either run zero or one time. It's run zero times if the Delay is never derefed.

Delays are also useful for a shared resource between threads.

1_delay_threadpool.clj

;; 
;; abstract example
;;
;; make a delay that initializes a shared resource
;; if we don't put it in a Delay, the resource will
;; be initialized during compilation
(def resource (delay (initialize-shared-resource)))

;; start 100 threads that use the resource
(defn -main []
  (dotimes [x 100]
    (doto (Thread. (fn []
                     (let [resource @resource]
                       ;; use the resource here ...
                       ))
                   .start)))

  
;;
;; c3p0 数据源， 特征: 执行0或者1次，缓存
;;
(ns clojure-tutorial.jdbc.c3p0
  (:import com.mchange.v2.c3p0.ComboPooledDataSource))

(require '[clojure.java.jdbc :as j])

(defn pool
  [spec]
  (let [cpds (doto (ComboPooledDataSource.)
               (.setDriverClass (:classname spec))
               (.setJdbcUrl (str "jdbc:" (:subprotocol spec) ":" (:subname spec)))
               (.setUser (:user spec))
               (.setPassword (:password spec))
               ;; expire excess connections after 30 minutes of inactivity:
               (.setMaxIdleTimeExcessConnections (* 30 60))
               ;; expire connections after 3 hours of inactivity:
               (.setMaxIdleTime (* 3 60 60)))]
    {:datasource cpds}))

;Now you can create a single connection pool:
(def db-spec {:classname   "com.mysql.jdbc.Driver"
               :subprotocol "mysql"
               :subname     "//10.4.7.15:3306/clojure_test"
               :user        "root"
               :password    "csst"})
;; 这里
(def pooled-db (delay (pool db-spec)))
(defn db-connection [] @pooled-db)
(j/insert! @pooled-db :fruit
           {:name "Apple" :appearance "rosy" :cost 24}
           {:name "Orange" :appearance "round" :cost 49})
;And then call (db-connection) wherever you need access to it. If you're using a component lifecycle for your application, such as Stuart Sierra has advocated, you won't need pooled-db or db-connection, you'll just create (pool db-spec) as part of your application's initialization and pass it around as part of your system configuration
(def pooled-db (pool db-spec))
(j/insert! pooled-db :fruit
           {:name "Apple" :appearance "rosy" :cost 24}
           {:name "Orange" :appearance "round" :cost 49})