{"id":8759,"date":"2023-11-28T20:39:26","date_gmt":"2023-11-28T12:39:26","guid":{"rendered":"https:\/\/wx.kaifamiao.info\/?p=8759"},"modified":"2023-11-28T20:40:19","modified_gmt":"2023-11-28T12:40:19","slug":"java-zhong-wei-shen-me-xu-yao-po-huai-shuang-qin-w","status":"publish","type":"post","link":"http:\/\/wx.kaifamiao.info\/index.php\/2023\/11\/28\/java-zhong-wei-shen-me-xu-yao-po-huai-shuang-qin-w\/","title":{"rendered":"Java\u4e2d\u4e3a\u4ec0\u4e48\u9700\u8981\u7834\u574f\u53cc\u4eb2\u59d4\u6d3e\uff1f"},"content":{"rendered":"

\u56e0\u4e3a\u5728\u67d0\u4e9b\u60c5\u51b5\u4e0b\u7236\u7c7b\u52a0\u8f7d\u5668\u9700\u8981\u59d4\u6258\u5b50\u7c7b\u52a0\u8f7d\u5668\u53bb\u52a0\u8f7dclass\u6587\u4ef6\u3002\u53d7\u5230\u52a0\u8f7d\u8303\u56f4\u7684\u9650\u5236,\u7236\u7c7b\u52a0\u8f7d\u5668\u65e0\u6cd5\u52a0\u8f7d\u5230\u9700\u8981\u7684\u6587\u4ef6,\u4ee5Driver\u63a5\u53e3\u4e3a\u4f8b,\u7531\u4e8eDriver\u63a5\u53e3\u5b9a\u4e49\u5728jdk\u5f53\u4e2d\u7684\uff0c\u800c\u5176\u5b9e\u73b0\u7531\u5404\u4e2a\u6570\u636e\u5e93\u7684\u670d\u52a1\u5546\u6765\u63d0\u4f9b\u3002
\n\u6bd4\u5982mysql\u7684\u5c31\u5199\u4e86MySQL Connector\uff0c\u90a3\u4e48\u95ee\u9898\u5c31\u6765\u4e86,DriverManager\uff08\u4e5f\u7531jdk\u63d0\u4f9b\uff09\u8981\u52a0\u8f7d\u5404\u4e2a\u5b9e\u73b0\u4e86Driver\u63a5\u53e3\u7684\u5b9e\u73b0\u7c7b,\u7136\u540e\u8fdb\u884c\u7ba1\u7406,\u4f46\u662fDriverManager\u7531\u542f\u52a8\u7c7b\u52a0\u8f7d\u5668\u52a0\u8f7d\uff0c\u53ea\u80fd\u8bb0\u8f7dJAVA_HOME \u7684lib\u4e0b\u6587\u4ef6\uff0c\u800c\u5176\u5b9e\u73b0\u662f\u7531\u670d\u52a1\u5546\u63d0\u4f9b\u7684\uff0c\u7531\u7cfb\u7edf\u7c7b\u52a0\u8f7d\u5668\u52a0\u8f7d\uff0c\u8fd9\u4e2a\u65f6\u5019\u5c31\u9700\u8981\u542f\u52a8\u7c7b\u52a0\u8f7d\u5668\u6765\u59d4\u6258\u5b50\u7c7b\u6765\u52a0\u8f7dDriver\u5b9e\u73b0\uff0c\u4ece\u800c\u7834\u574f\u4e86\u53cc\u4eb2\u59d4\u6d3e\uff0c\u8fd9\u91cc\u4ec5\u4ec5\u662f\u4e3e\u4e86\u7834\u574f\u53cc\u4eb2\u59d4\u6d3e\u7684\u5176\u4e2d\u4e00\u4e2a\u60c5\u51b5
\n\u6211\u4eec\u7ed3\u5408Driver\u6765\u770b\u4e00\u4e0b\u5728spi(Service Provider Interface)\u4e2d\u5982\u4f55\u5b9e\u73b0\u7834\u574f\u53cc\u4eb2\u59d4\u6d3e\u3002
\n\u5148\u4eceDriverManager\u5f00\u59cb\u770b\uff0c\u5e73\u65f6\u6211\u4eec\u901a\u8fc7DriverManager\u6765\u83b7\u53d6\u6570\u636e\u5e93\u7684Connection:
\nString url = "jdbc:mysql:\/\/localhost:3306\/testdb";
\nConnection conn = java.sql.DriverManager.getConnection(url,"root","root");
\n\u5728\u8c03\u7528DriverManager\u7684\u65f6\u5019,\u4f1a\u5148\u521d\u59cb\u5316\u7c7b,\u8c03\u7528\u5176\u4e2d\u7684\u9759\u6001\u5757:
\nstatic{
\nloadInitialDrivers();
\nprintln("JDBC DriverManager initialized");
\n}
\nprivate static void loadInitialDrivers(){
\n…
\n\/\/\u52a0\u8f7dDriver\u7684\u5b9e\u73b0\u7c7b
\nAccessController.doPrivileged(new PrivilegedAction(){
\npublic void run(){
\nServiceLoader loadedDrivers = ServiceLoader.load(Driver.class);
\nIterator driversIterator = loadedDrivers.iterator();
\ntry{
\nwhile(driversIterator.hasNext()){
\ndriversIterator.next();
\n}
\n} catch(Throwable t){
\n}
\nreturn null;
\n}
\n});
\n…
\n}
\n\u4e3a\u4e86\u8282\u7ea6\u7a7a\u95f4,\u7b14\u8005\u7701\u7565\u4e86\u4e00\u90e8\u5206\u7684\u4ee3\u7801\uff0c\u91cd\u70b9\u6765\u770b\u4e00\u4e0b
\nServiceLoader.load(Driver.class):
\npublic static ServiceLoader load(Class service){
\n\/\/\u83b7\u53d6\u5f53\u524d\u7ebf\u7a0b\u4e2d\u7684\u4e0a\u4e0b\u6587\u7c7b\u52a0\u8f7d\u5668
\nClassLoader cl = Thread.currentThread().getContextClassLoader();
\nreturn ServiceLoader.load(service,cl);
\n}
\n\u53ef\u4ee5\u770b\u5230\uff0cload\u65b9\u6cd5\u8c03\u7528\u83b7\u53d6\u4e86\u5f53\u524d\u7ebf\u7a0b\u4e2d\u7684\u4e0a\u4e0b\u6587\u7c7b\u52a0\u8f7d\u5668\uff0c\u90a3\u4e48\u4e0a\u4e0b\u6587\u7c7b\u52a0\u8f7d\u5668\u653e\u7684\u662f\u4ec0\u4e48\u52a0\u8f7d\u5668\u5462?
\npublic Launcher(){
\n…
\ntry{
\nthis.loader = Launcher.AppClassLoader.getAppClassLoader(var1);
\n}catch (IOException var9){
\nthrow new InternalError("Could not create application class loader", var9);
\n}
\nThread.currentThread().setContextClassLoader(this.loader);
\n…
\n}
\n\u5728sun.misc.Launcher\u4e2d\uff0c\u6211\u4eec\u627e\u5230\u4e86\u7b54\u6848\uff0c\u5728Launcher\u521d\u59cb\u5316\u7684\u65f6\u5019\uff0c\u4f1a\u83b7\u53d6AppClassLoader\uff0c\u7136\u540e\u5c06\u5176\u8bbe\u7f6e\u4e3a\u4e0a\u4e0b\u6587\u7c7b\u52a0\u8f7d\u5668\uff0c\u800c\u8fd9\u4e2aAppClassLoader,\u5c31\u662f\u4e4b\u524d\u4e0a\u6587\u63d0\u5230\u7684\u7cfb\u7edf\u7c7b\u52a0\u8f7d\u5668Application ClassLoader,\u6240\u4ee5\u4e0a\u4e0b\u6587\u7c7b\u52a0\u8f7d\u5668\u9ed8\u8ba4\u60c5\u51b5\u4e0b\u5c31\u662f\u7cfb\u7edf\u52a0\u8f7d\u5668\u3002
\n\u7ee7\u7eed\u6765\u770b\u4e0bServiceLoader.load(service,cl):
\npublic static ServiceLoader load(Class service,ClassLoader loader){
\nreturn new ServiceLoader<>(service,loader);
\n}
\nprivate ServiceLoader(Class svc,ClassLoader cl){
\nservice = 0bjects.requireNonNull(svc,"Service interface cannot be null");
\n\/\/ClassLoader.getSystemClassLoader()\u8fd4\u56de\u7684\u4e5f\u662f\u7cfb\u7edf\u7c7b\u52a0\u8f7d\u5668
\nloader = (cl == null)? ClassLoader.getSystemClassLoader():cl;
\nacc = (System.getSecurityManager() != null)? AccessController.getContext():null;
\nreload();
\n}
\npublic void reload(){
\nproviders.clear();
\nlookupIterator = new LazyIterator(service,loader);
\n}
\n\u4e0a\u9762\u8fd9\u6bb5\u5c31\u4e0d\u89e3\u91ca\u4e86\uff0c\u6bd4\u8f83\u7b80\u5355\uff0c\u7136\u540e\u5c31\u662f\u770bLazyIterator\u8fed\u4ee3\u5668:
\nprivate class LazyIterator implements Iterator{
\n\/\/ ServiceLoader\u7684iterator()\u65b9\u6cd5\u6700\u540e\u8c03\u7528\u7684\u8d70\u8fd9\u4e2a\u8fed\u4ee3\u5668\u91cc\u7684next
\npublic S next(){
\nif (acc == nul1){
\nreturn nextService();
\n}else {
\nPrivilegedAction action = new PrivilegedAction(){
\npublic S run() { return nextService();}
\n};
\nreturn AccessController.doPrivileged(action, acc);
\n}
\n}
\nprivate S nextService(){
\nif (!hasNextService())
\nthrow new NoSuchElementException();
\nString cn = nextName;
\nnextName = null;
\nClass<?> c = null;
\n\/\/\u6839\u636e\u540d\u5b57\u6765\u52a0\u8f7d\u7c7b
\ntry{
\nc = Class.forName(cn,false,loader);
\n}catch (ClassNotFoundException x){
\nfail(service,
\n"Provider " + cn +" not found");
\n}
\nif (!service.isAssignableFrom(c)){
\nfail (service,
\n"Provider " + cn + " not a subtype");
\n}
\ntry {
\nS p = service.cast(c.newInstance());
\nproviders.put(cn,p);
\nreturn p;
\n}catch (Throwable x){
\nfail(service,
\n"Provider " + cn+ "could not be instantiated",x);
\n}
\nthrow new Error(); \/\/ This cannot happen
\n}
\npublic boolean hasNext(){
\nif (acc == null){
\nreturn hasNextService();
\n}else{
\nPrivilegedAction action = new PrivilegedAction(){
\npublic Boolean run(){return hasNextService();}
\n};
\nreturn AccessController.doPrivileged(action, acc);
\n}
\n}
\nprivate boolean hasNextService(){
\nif (nextName != null){
\nreturn true;
\n}
\nif (configs == nul1){
\ntry {
\n\/\/\u5728classpath\u4e0b\u67e5\u627eMETA-INF\/services\/java.sql.Driver\u540d\u5b57\u7684\u6587\u4ef6\u5939
\n\/\/ private static final String PREFIX = "META-INF\/services\/";
\nString fullName = PREFIX + service.getName();
\nif (loader == null)
\nconfigs = ClassLoader.getSystemResources(fullName);
\nelse
\nconfigs = loader.getResources(ful1Name);
\n} catch (IOException x){
\nfail(service, "Error locating configuration files", x);
\n}
\n}
\nwhile((pending == null) || !pending.hasNext()){
\nif(!configs.hasMoreElements()){
\nreturn false;
\n}
\npending = parse(service,configs.nextElement());
\n}
\nnextName = pending.next();
\nreturn true;
\n}
\n}
\n\u597d\u4e86\uff0c\u8fd9\u91cc\u57fa\u672c\u5c31\u5dee\u4e0d\u591a\u5b8c\u6210\u6574\u4e2a\u6d41\u7a0b\u4e86\uff0c\u4e00\u8d77\u8d70\u4e00\u904d:
\n\"\"<\/p>\n","protected":false},"excerpt":{"rendered":"

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