{"id":2771,"date":"2018-05-06T14:42:54","date_gmt":"2018-05-06T06:42:54","guid":{"rendered":"http:\/\/www.xttblog.com\/?p=2771"},"modified":"2018-05-06T14:42:54","modified_gmt":"2018-05-06T06:42:54","slug":"java-%e8%87%aa%e5%ae%9a%e4%b9%89%e7%b1%bb%e5%8a%a0%e8%bd%bd%e5%99%a8%e6%95%99%e7%a8%8b","status":"publish","type":"post","link":"https:\/\/www.xttblog.com\/?p=2771","title":{"rendered":"Java \u81ea\u5b9a\u4e49\u7c7b\u52a0\u8f7d\u5668\u6559\u7a0b"},"content":{"rendered":"

\n\t\u9664\u4e86\u5728\u9762\u8bd5\u4e2d\u9047\u5230\u7c7b\u7684\u52a0\u8f7d\u5668\u7684\u6982\u7387\u4f1a\u9ad8\u5916\uff0c\u5728\u5b9e\u9645\u7684\u5de5\u4f5c\u4e2d\u5f88\u5c11\u63a5\u89e6\u3002\u4f46\u662f\u4e00\u4e2a\u7a0b\u5e8f\u5458\u60f3\u8981\u6210\u957f\u4e3a\u5927\u725b\u5c31\u5fc5\u987b\u5bf9\u4e00\u4e9b JVM \u7684\u5e95\u5c42\u8bbe\u8ba1\u6709\u4e9b\u4e86\u89e3\u3002\u5728\u6b64\u57fa\u7840\u4e0a\u6211\u4eec\u9605\u8bfb\u4e00\u4e9b\u6e90\u7801\u548c\u6846\u67b6\u4f1a\u663e\u5f97\u66f4\u8f7b\u677e\u3002\n<\/p>\n

\n\t\u597d\u4e86\u5e9f\u8bdd\u4e0d\u591a\u8bf4\uff0c\u6211\u4eec\u63a5\u7740\u524d\u9762\u7684\u6587\u7ae0\uff0c\u4e58\u70ed\u6253\u94c1\u3002\u6765\u5b9e\u73b0\u4e00\u4e2a Java \u81ea\u5b9a\u4e49\u7c7b\u52a0\u8f7d\u5668\u5427\u3002\n<\/p>\n

\n\t\u8981\u5b9e\u73b0 Java \u81ea\u5b9a\u4e49\u7684\u7c7b\u52a0\u8f7d\u5668\uff0c\u6211\u4eec\u9700\u8981\u7ee7\u627f ClassLoader \u3002\u5e76\u4e14\u9700\u8981\u4e86\u89e3Java\u7684\u53cc\u4eb2\u59d4\u6d3e\u6a21\u578b<\/a>\u3002\n<\/p>\n

\n\tloadClass
\n<\/h2>\n

\n\tloadClass\u9ed8\u8ba4\u5b9e\u73b0\u5982\u4e0b\uff1a\n<\/p>\n

\r\npublic Class<?> loadClass(String name) throws ClassNotFoundException {\r\n    return loadClass(name, false);\r\n}<\/pre>\n
\n\t\u518d\u770b\u770bloadClass(String name, boolean resolve)\u51fd\u6570\uff1a\n<\/p>\n
\r\nprotected Class<?> loadClass(String name, boolean resolve)\r\n    throws ClassNotFoundException{\r\n    synchronized (getClassLoadingLock(name)) {\r\n        \/\/ First, check if the class has already been loaded\r\n        Class c = findLoadedClass(name);\r\n        if (c == null) {\r\n            long t0 = System.nanoTime();\r\n            try {\r\n                if (parent != null) {\r\n                    c = parent.loadClass(name, false);\r\n                } else {\r\n                    c = findBootstrapClassOrNull(name);\r\n                }\r\n            } catch (ClassNotFoundException e) {\r\n                \/\/ ClassNotFoundException thrown if class not found\r\n                \/\/ from the non-null parent class loader\r\n            }\r\n\t\t\t\/\/ \u4e1a\u4f59\u8349\uff1awww.xttblog.com\r\n            if (c == null) {\r\n                \/\/ If still not found, then invoke findClass in order\r\n                \/\/ to find the class.\r\n                long t1 = System.nanoTime();\r\n                c = findClass(name);\r\n                \/\/ this is the defining class loader; record the stats\r\n                sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);\r\n                sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);\r\n                sun.misc.PerfCounter.getFindClasses().increment();\r\n            }\r\n        }\r\n        if (resolve) {\r\n            resolveClass(c);\r\n        }\r\n        return c;\r\n    }\r\n}<\/pre>\n
\n\t\u4ece\u4e0a\u9762\u4ee3\u7801\u53ef\u4ee5\u660e\u663e\u770b\u51fa\uff0cloadClass(String, boolean)<\/code>\u51fd\u6570\u5373\u5b9e\u73b0\u4e86\u53cc\u4eb2\u59d4\u6d3e\u6a21\u578b\uff01\u6574\u4e2a\u5927\u81f4\u8fc7\u7a0b\u5982\u4e0b\uff1a\n<\/p>\n
    \n
  1. \n\t\t\u9996\u5148\uff0c\u68c0\u67e5\u4e00\u4e0b\u6307\u5b9a\u540d\u79f0\u7684\u7c7b\u662f\u5426\u5df2\u7ecf\u52a0\u8f7d\u8fc7\uff0c\u5982\u679c\u52a0\u8f7d\u8fc7\u4e86\uff0c\u5c31\u4e0d\u9700\u8981\u518d\u52a0\u8f7d\uff0c\u76f4\u63a5\u8fd4\u56de\u3002\n\t<\/li>\n
  2. \n\t\t\u5982\u679c\u6b64\u7c7b\u6ca1\u6709\u52a0\u8f7d\u8fc7\uff0c\u90a3\u4e48\uff0c\u518d\u5224\u65ad\u4e00\u4e0b\u662f\u5426\u6709\u7236\u52a0\u8f7d\u5668\uff1b\u5982\u679c\u6709\u7236\u52a0\u8f7d\u5668\uff0c\u5219\u7531\u7236\u52a0\u8f7d\u5668\u52a0\u8f7d\uff08\u5373\u8c03\u7528parent.loadClass(name, false);\uff09.\u6216\u8005\u662f\u8c03\u7528bootstrap\u7c7b\u52a0\u8f7d\u5668\u6765\u52a0\u8f7d\u3002\n\t<\/li>\n
  3. \n\t\t\u5982\u679c\u7236\u52a0\u8f7d\u5668\u53cabootstrap\u7c7b\u52a0\u8f7d\u5668\u90fd\u6ca1\u6709\u627e\u5230\u6307\u5b9a\u7684\u7c7b\uff0c\u90a3\u4e48\u8c03\u7528\u5f53\u524d\u7c7b\u52a0\u8f7d\u5668\u7684findClass\u65b9\u6cd5\u6765\u5b8c\u6210\u7c7b\u52a0\u8f7d\u3002\n\t<\/li>\n<\/ol>\n
    \n\t\u8bdd\u53e5\u8bdd\u8bf4\uff0c\u5982\u679c\u81ea\u5b9a\u4e49\u7c7b\u52a0\u8f7d\u5668\uff0c\u5c31\u5fc5\u987b\u91cd\u5199findClass\u65b9\u6cd5\uff01\n<\/p>\n
    \n\tfindClass
    \n<\/h2>\n
    \n\tfindClass\u7684\u9ed8\u8ba4\u5b9e\u73b0\u5982\u4e0b\uff1a\n<\/p>\n
    \r\nprotected Class<?> findClass(String name) throws ClassNotFoundException {\r\n    throw new ClassNotFoundException(name);\r\n}<\/pre>\n
    \n\t\u53ef\u4ee5\u770b\u51fa\uff0c\u62bd\u8c61\u7c7bClassLoader\u7684findClass\u51fd\u6570\u9ed8\u8ba4\u662f\u629b\u51fa\u5f02\u5e38\u7684\u3002\u800c\u524d\u9762\u6211\u4eec\u77e5\u9053\uff0cloadClass\u5728\u7236\u52a0\u8f7d\u5668\u65e0\u6cd5\u52a0\u8f7d\u7c7b\u7684\u65f6\u5019\uff0c\u5c31\u4f1a\u8c03\u7528\u6211\u4eec\u81ea\u5b9a\u4e49\u7684\u7c7b\u52a0\u8f7d\u5668\u4e2d\u7684findeClass\u51fd\u6570\uff0c\u56e0\u6b64\u6211\u4eec\u5fc5\u987b\u8981\u5728loadClass\u8fd9\u4e2a\u51fd\u6570\u91cc\u9762\u5b9e\u73b0\u5c06\u4e00\u4e2a\u6307\u5b9a\u7c7b\u540d\u79f0\u8f6c\u6362\u4e3aClass\u5bf9\u8c61.\n<\/p>\n
    \n\t\u5982\u679c\u662f\u662f\u8bfb\u53d6\u4e00\u4e2a\u6307\u5b9a\u7684\u540d\u79f0\u7684\u7c7b\u4e3a\u5b57\u8282\u6570\u7ec4\u7684\u8bdd\uff0c\u8fd9\u5f88\u597d\u529e\u3002\u4f46\u662f\u5982\u4f55\u5c06\u5b57\u8282\u6570\u7ec4\u8f6c\u4e3aClass\u5bf9\u8c61\u5462\uff1f\u5f88\u7b80\u5355\uff0cJava\u63d0\u4f9b\u4e86defineClass\u65b9\u6cd5\uff0c\u901a\u8fc7\u8fd9\u4e2a\u65b9\u6cd5\uff0c\u5c31\u53ef\u4ee5\u628a\u4e00\u4e2a\u5b57\u8282\u6570\u7ec4\u8f6c\u4e3aClass\u5bf9\u8c61\u3002\n<\/p>\n
    \n\tdefineClass
    \n<\/h2>\n
    \n\tdefineClass\u4e3b\u8981\u7684\u529f\u80fd\u662f\uff1a\u5c06\u4e00\u4e2a\u5b57\u8282\u6570\u7ec4\u8f6c\u4e3aClass\u5bf9\u8c61\uff0c\u8fd9\u4e2a\u5b57\u8282\u6570\u7ec4\u662fclass\u6587\u4ef6\u8bfb\u53d6\u540e\u6700\u7ec8\u7684\u5b57\u8282\u6570\u7ec4\u3002\u5982\uff0c\u5047\u8bbeclass\u6587\u4ef6\u662f\u52a0\u5bc6\u8fc7\u7684\uff0c\u5219\u9700\u8981\u89e3\u5bc6\u540e\u4f5c\u4e3a\u5f62\u53c2\u4f20\u5165defineClass\u51fd\u6570\u3002\n<\/p>\n
    \n\tdefineClass\u9ed8\u8ba4\u5b9e\u73b0\u5982\u4e0b\uff1a\n<\/p>\n
    \r\nprotected final Class<?> defineClass(String name, byte[] b, int off, int len)\r\n\tthrows ClassFormatError  {\r\n\treturn defineClass(name, b, off, len, null);\r\n}<\/pre>\n
    \n\t\u51fd\u6570\u8c03\u7528\u8fc7\u7a0b
    \n<\/h2>\n
    \n\t\u51fd\u6570\u8c03\u7528\u8fc7\u7a0b\uff0c\u5982\u4e0b\u56fe\u6240\u793a\uff1a\n<\/p>\n
    \n\t\"Java\n<\/p>\n
    \n\t\u901a\u5e38\u60c5\u51b5\u4e0b\uff0c\u6211\u4eec\u90fd\u662f\u76f4\u63a5\u4f7f\u7528\u7cfb\u7edf\u7c7b\u52a0\u8f7d\u5668\u3002\u4f46\u662f\uff0c\u6709\u7684\u65f6\u5019\uff0c\u6211\u4eec\u4e5f\u9700\u8981\u81ea\u5b9a\u4e49\u7c7b\u52a0\u8f7d\u5668\u3002\u6bd4\u5982\u5e94\u7528\u662f\u901a\u8fc7\u7f51\u7edc\u6765\u4f20\u8f93 Java\u7c7b\u7684\u5b57\u8282\u7801\uff0c\u4e3a\u4fdd\u8bc1\u5b89\u5168\u6027\uff0c\u8fd9\u4e9b\u5b57\u8282\u7801\u7ecf\u8fc7\u4e86\u52a0\u5bc6\u5904\u7406\uff0c\u8fd9\u65f6\u7cfb\u7edf\u7c7b\u52a0\u8f7d\u5668\u5c31\u65e0\u6cd5\u5bf9\u5176\u8fdb\u884c\u52a0\u8f7d\uff0c\u8fd9\u6837\u5219\u9700\u8981\u81ea\u5b9a\u4e49\u7c7b\u52a0\u8f7d\u5668\u6765\u5b9e\u73b0\u3002\u81ea\u5b9a\u4e49\u7c7b\u52a0\u8f7d\u5668\u4e00\u822c\u90fd\u662f\u7ee7\u627f\u81eaClassLoader\u7c7b\uff0c\u4ece\u4e0a\u9762\u5bf9loadClass\u65b9\u6cd5\u6765\u5206\u6790\u6765\u770b\uff0c\u6211\u4eec\u53ea\u9700\u8981\u91cd\u5199 findClass \u65b9\u6cd5\u5373\u53ef\u3002\u4e0b\u9762\u6211\u4eec\u901a\u8fc7\u4e00\u4e2a\u793a\u4f8b\u6765\u6f14\u793a\u81ea\u5b9a\u4e49\u7c7b\u52a0\u8f7d\u5668\u7684\u6d41\u7a0b\uff1a\n<\/p>\n
    \r\npackage com.xttblog.classloader;\r\nimport java.io.*;\r\npublic class MyClassLoader extends ClassLoader {\r\n    private String root;\r\n\t\/\/ \u4e1a\u4f59\u8349\uff1awww.xttblog.com\r\n    protected Class<?> findClass(String name) throws ClassNotFoundException {\r\n        byte[] classData = loadClassData(name);\r\n        if (classData == null) {\r\n            throw new ClassNotFoundException();\r\n        } else {\r\n            return defineClass(name, classData, 0, classData.length);\r\n        }\r\n    }\r\n    private byte[] loadClassData(String className) {\r\n        String fileName = root + File.separatorChar\r\n                + className.replace('.', File.separatorChar) + ".class";\r\n        try {\r\n            InputStream ins = new FileInputStream(fileName);\r\n            ByteArrayOutputStream baos = new ByteArrayOutputStream();\r\n            int bufferSize = 1024;\r\n            byte[] buffer = new byte[bufferSize];\r\n            int length = 0;\r\n            while ((length = ins.read(buffer)) != -1) {\r\n                baos.write(buffer, 0, length);\r\n            }\r\n            return baos.toByteArray();\r\n        } catch (IOException e) {\r\n            e.printStackTrace();\r\n        }\r\n        return null;\r\n    }\r\n    public String getRoot() {\r\n        return root;\r\n    }\r\n    public void setRoot(String root) {\r\n        this.root = root;\r\n    }\r\n    public static void main(String[] args)  {\r\n        MyClassLoader classLoader = new MyClassLoader();\r\n        classLoader.setRoot("E:\\\\temp");\r\n        Class<?> testClass = null;\r\n        try {\r\n            testClass = classLoader.loadClass("com.xttblog.classloader.Test2");\r\n            Object object = testClass.newInstance();\r\n            System.out.println(object.getClass().getClassLoader());\r\n        } catch (ClassNotFoundException e) {\r\n            e.printStackTrace();\r\n        } catch (InstantiationException e) {\r\n            e.printStackTrace();\r\n        } catch (IllegalAccessException e) {\r\n            e.printStackTrace();\r\n        }\r\n    }\r\n}<\/pre>\n
    \n\t\u81ea\u5b9a\u4e49\u7c7b\u52a0\u8f7d\u5668\u7684\u6838\u5fc3\u5728\u4e8e\u5bf9\u5b57\u8282\u7801\u6587\u4ef6\u7684\u83b7\u53d6\uff0c\u5982\u679c\u662f\u52a0\u5bc6\u7684\u5b57\u8282\u7801\u5219\u9700\u8981\u5728\u8be5\u7c7b\u4e2d\u5bf9\u6587\u4ef6\u8fdb\u884c\u89e3\u5bc6\u3002\u7531\u4e8e\u8fd9\u91cc\u53ea\u662f\u6f14\u793a\uff0c\u6211\u5e76\u672a\u5bf9class\u6587\u4ef6\u8fdb\u884c\u52a0\u5bc6\uff0c\u56e0\u6b64\u6ca1\u6709\u89e3\u5bc6\u7684\u8fc7\u7a0b\u3002\u8fd9\u91cc\u6709\u51e0\u70b9\u9700\u8981\u6ce8\u610f\uff1a\n<\/p>\n