6 自定义类加载器
6.1 自定义类加载器的应用场景
通过合理使用上述 JVM 自带的各个类加载器，一般就已经能够满足绝大多情况下类或资源的加载需求了。 但在某些特殊情况下，我们仍然需要自定义类加载器，这些场景主要有：
- 辅助修改已有的字节码 bytecode, 比如 weaving agents；
- 加载本地磁盘文件系统之外的网络上的类或资源，比如浏览器加载远程web服务器上的 applet；
6.2 如何自定义类加载器
自定义类加载器,只需要继承类 ClassLoader 并复写方法 findClass() 即可。 以下示例代码，自定义了一个类加载器，来从指定的文件中加载 byte array 并构建类：
public class CustomClassLoader extends ClassLoader { @Override public Class findClass(String name) throws ClassNotFoundException { byte[] b = loadClassFromFile(name); return defineClass(name, b, 0, b.length); } private byte[] loadClassFromFile(String fileName) { InputStream inputStream = getClass().getClassLoader().getResourceAsStream( fileName.replace('.', File.separatorChar) + ".class"); byte[] buffer; ByteArrayOutputStream byteStream = new ByteArrayOutputStream(); int nextValue = 0; try { while ( (nextValue = inputStream.read()) != -1 ) { byteStream.write(nextValue); } } catch (IOException e) { e.printStackTrace(); } buffer = byteStream.toByteArray(); return buffer; } }
7 相关测试类
package com.hundsun; import org.apache.hadoop.conf.Configuration; import org.apache.hive.jdbc.HiveDriver; import java.io.File; import java.net.URISyntaxException; import java.net.URL; import java.net.URLClassLoader; import java.sql.*; import java.util.ArrayList; import java.util.List; /** * @author: liming * @date: 2021/8/28 20:12 * @description: */ public class ClassLoaderTest { private static String dataSourceDriverPath; private static String classPath = "E:\\git\\sparkTest\\target"; public static void main(String[] args) { try { // dataSourceDriverPath = args[0]; dataSourceDriverPath = "D:\\software\\数据源驱动-202101.03.000\\drivers\\hive-cdh6\\lib"; // System.out.println("驱动路径:" + dataSourceDriverPath); System.out.println("ClassLoaderTest.class.getClassLoader():" + ClassLoaderTest.class.getClassLoader().toString()); // System.out.println("ClassLoaderTest.class.getClassLoader().getParent():" + ClassLoaderTest.class.getClassLoader().getParent().toString()); // System.out.println("ClassLoaderTest.class.getClassLoader().getParent().getParent():" + ClassLoaderTest.class.getClassLoader().getParent().getParent()); // System.out.println("ClassLoaderTest.class.getClassLoader().getParent().getParent().getParent():" + ClassLoaderTest.class.getClassLoader().getParent().getParent().getParent()); jdbcDriverTest(); getDriverUsingContextClassLoader(); getDriverUsingUrlClassLoader(); getClassUsingContextClassLoader(classPath); } catch (Exception e) { System.out.println(e.getMessage()); System.out.println(e.getCause()); e.printStackTrace(); } } public static void jdbcDriverTest(){ try{ Class<?> driverManagerClass = Class.forName("java.sql.DriverManager"); Class<?> driverClass = Class.forName("java.sql.Driver"); Class<?> oracleDriverClass = Class.forName("oracle.jdbc.driver.OracleDriver"); System.out.println("driverManagerClass.getClassLoader():" + driverManagerClass.getClassLoader()); System.out.println("driverClass.getClassLoader():" + driverClass.getClassLoader()); System.out.println("oracleDriverClass.getClassLoader():" + oracleDriverClass.getClassLoader()); // Connection con=DriverManager.getConnection("jdbc:oracle:thin:@//10.20.29.239:1521/orclpdb1","hs_cic","hundsun"); Connection con= DriverManager.getConnection( "jdbc:oracle:thin:@//10.20.23.215:1521/orcl","hs_dap","hundsun"); System.out.println(con.getAutoCommit()); con.setAutoCommit(false); System.out.println(con.getAutoCommit()); Statement stmt=con.createStatement(); ResultSet rs=stmt.executeQuery("select count(1) from HS_DAP.DAP_DATA_TYPE_LIMING"); while (rs.next()) { System.out.println(rs.getInt(1)); // System.out.println(rs.getString(1) + " " + rs.getInt(2) + " " + rs.getString(3)); } PreparedStatement ps = con.prepareStatement("delete from HS_DAP.DAP_DATA_TYPE_LIMING where HIVE_DATA_TYPE = ?"); ps.setString(1,"DATE"); ps.execute(); Thread.sleep(1 * 60 * 1000); con.close(); }catch(Exception e){ System.out.println(e);} } public static Driver getDriverUsingUrlClassLoader() throws URISyntaxException { String driverPath = dataSourceDriverPath; List<File> files = scanDir(driverPath); System.out.println(files.get(0).toURI()); Class<?> driver = null; Class<?> configuration = null; try { URL[] urls = new URL[files.size()]; for (int i = 0; i < files.size(); i++) { urls[i] = files.get(i).toURI().toURL(); // System.out.println(files.get(i).toURI()); } System.out.println("Thread.currentThread().getContextClassLoader():" + Thread.currentThread().getContextClassLoader()); URLClassLoader urlClassLoader = new URLClassLoader(urls); System.out.println("urlClassLoader:" + urlClassLoader); URL resource = urlClassLoader.getResource("core-site-test.xml"); String resourceURI = resource.toURI().toString(); System.out.println("resourceURI:" + resourceURI); System.out.println("resource.getPath():" + resource.getPath()); driver = urlClassLoader.loadClass("org.apache.hive.jdbc.HiveDriver"); System.out.println("org.apache.hive.jdbc.HiveDriver.class.getClassLoader():" + HiveDriver.class.getClassLoader().toString()); System.out.println("org.apache.hive.jdbc.HiveDriver.class.getClassLoader().getParent()：" + HiveDriver.class.getClassLoader().getParent().toString()); configuration = urlClassLoader.loadClass("org.apache.hadoop.conf.Configuration"); System.out.println("org.apache.hadoop.conf.Configuration.class.getClassLoader():" + Configuration.class.getClassLoader().toString()); System.out.println("org.apache.hadoop.conf.Configuration.class.getClassLoader().getParent()：" + Configuration.class.getClassLoader().getParent().toString()); System.out.println("Thread.currentThread().getContextClassLoader():" + Thread.currentThread().getContextClassLoader()); System.out.println("urlClassLoader.getParent():" + urlClassLoader.getParent()); Thread.currentThread().setContextClassLoader(urlClassLoader); System.out.println("Thread.currentThread().getContextClassLoader()" + Thread.currentThread().getContextClassLoader()); return (Driver) driver.newInstance(); } catch (Exception e) { System.out.println(e.getCause()); System.out.println(e.getMessage()); e.printStackTrace(); } return null; } public static Driver getDriverUsingContextClassLoader() throws URISyntaxException { String driverPath = dataSourceDriverPath; List<File> files = scanDir(driverPath); System.out.println(files.get(0).toURI()); Class<?> driver = null; Class<?> configuration = null; try { URL[] urls = new URL[files.size()]; for (int i = 0; i < files.size(); i++) { urls[i] = files.get(i).toURI().toURL(); // System.out.println(files.get(i).toURI()); } ClassLoader prevContextClassLoader = Thread.currentThread().getContextClassLoader(); System.out.println("prevContextClassLoader:" + prevContextClassLoader); // create a URLClassLoader with the thread ContextClassLoader as parent URLClassLoader urlContextClassLoader = new URLClassLoader(urls, prevContextClassLoader); System.out.println("urlContextClassLoader:" + urlContextClassLoader); URL resource = urlContextClassLoader.getResource("core-site-test.xml"); String resourceURI = resource.toURI().toString(); // System.out.println("g:" +resourceURI); System.out.println("resource.getPath():" + resource.getPath()); driver = urlContextClassLoader.loadClass("org.apache.hive.jdbc.HiveDriver"); System.out.println("org.apache.hive.jdbc.HiveDriver.class.getClassLoader():" + org.apache.hive.jdbc.HiveDriver.class.getClassLoader().toString()); System.out.println("org.apache.hive.jdbc.HiveDriver.class.getClassLoader().getParent()：" + org.apache.hive.jdbc.HiveDriver.class.getClassLoader().getParent().toString()); configuration = urlContextClassLoader.loadClass("org.apache.hadoop.conf.Configuration"); System.out.println("Thread.currentThread().getContextClassLoader():" + Thread.currentThread().getContextClassLoader()); System.out.println("org.apache.hadoop.conf.Configuration.class.getClassLoader():" + org.apache.hadoop.conf.Configuration.class.getClassLoader().toString()); System.out.println("org.apache.hadoop.conf.Configuration.class.getClassLoader().getParent()：" + org.apache.hadoop.conf.Configuration.class.getClassLoader().getParent().toString()); System.out.println("urlContextClassLoader.getParent():" + urlContextClassLoader.getParent()); // set the thread contextClassLoader to the newly created urlContextClassLoader Thread.currentThread().setContextClassLoader(urlContextClassLoader); System.out.println("Thread.currentThread().getContextClassLoader()" + Thread.currentThread().getContextClassLoader()); return (Driver) driver.newInstance(); } catch (Exception e) { System.out.println(e.getCause()); System.out.println(e.getMessage()); e.printStackTrace(); } return null; } public static void getClassUsingContextClassLoader(String classPath) throws URISyntaxException { List<File> files = scanDir(classPath); System.out.println(files.get(0).toURI()); try { URL[] urls = new URL[files.size()]; for (int i = 0; i < files.size(); i++) { urls[i] = files.get(i).toURI().toURL(); // System.out.println(files.get(i).toURI()); } ClassLoader prevContextClassLoader = Thread.currentThread().getContextClassLoader(); System.out.println("prevContextClassLoader:" + prevContextClassLoader); System.out.println("prevContextClassLoader.getParent():" + prevContextClassLoader.getParent()); System.out.println("prevContextClassLoader.getParent().getParent():" + prevContextClassLoader.getParent().getParent()); URLClassLoader urlContextClassLoader = new URLClassLoader(urls, prevContextClassLoader); System.out.println("urlContextClassLoader:" + urlContextClassLoader); System.out.println("ClassLoader.getSystemClassLoader():" + ClassLoader.getSystemClassLoader()); Thread.currentThread().setContextClassLoader(urlContextClassLoader); System.out.println("Thread.currentThread().getContextClassLoader()" + Thread.currentThread().getContextClassLoader()); System.out.println("Thread.currentThread().getContextClassLoader().getParent()" + Thread.currentThread().getContextClassLoader().getParent()); System.out.println("ClassLoader.getSystemClassLoader():" + ClassLoader.getSystemClassLoader()); // if you call Class.forName() without specifying the classloader, it will use the system classloader by default; // Class<?> claz1 = Class.forName("com.hundsun.SimpleApp"); // Class<?> claz1 = Class.forName("com.hundsun.SimpleApp",true,urlContextClassLoader); // if you call Class.forName() with the same class and same classloader multiple times, // the class will be loaded and initialized only once // Class.forName("com.hundsun.SimpleApp",true,urlContextClassLoader); // Class.forName("com.hundsun.SimpleApp",true,urlContextClassLoader); // ClassLoader.loadClass will only load the class but will not initialize it - the static code block will not be executed; Class claz1 = urlContextClassLoader.loadClass("com.hundsun.SimpleApp"); System.out.println("claz1.getClassLoader()" + claz1.getClassLoader()); } catch (Exception e) { System.out.println(e.getCause()); System.out.println(e.getMessage()); e.printStackTrace(); } } /** * 扫描lib下面的所有jar包 * * @return */ private static List<File> scanDir(String path) { List<File> list = new ArrayList<>(); File[] files = new File(path).listFiles(); for (File f : files) { if (f.isFile() && f.getName().endsWith(".jar")) { list.add(f); } if (f.listFiles() != null) { for (File file : f.listFiles()) { if (file.isFile() && file.getName().endsWith(".jar")) { list.add(file); } } } } return list; } }
8 知识总结
- JVM 类加载器是JRE的一部分，负责在 JVM 程序运行时，动态加载 JAVA 类到 JVM 内存中；
- JRE 中内置的类加载器有 sun.misc.Launcher.AppClassLoader/sun.misc.Launcher.ExtClassLoader/java.net.URLClassLoader/bootstrap class loader 等；
- 类加载器加载类，遵循了双亲委派模型，其核心思想是：
- 当类加载器收到一个类或资源的加载请求后，会首先搜索该类或资源是否已经被加载到内存中了，如果已经被加载了的话，不会重复加载该类或资源；（在内存中搜索类或资源时，一样会遵循类的可见性）；
- 如果没有被加载，则会将加载请求委派给父级加载器去加载；
- 父级加载器一样遵循向上委派机制，将加载请求委派给其父级加载器去加载，通过这种向上传导关系，所有的类加载请求，最终都会被传入到启动类加载器(Bootstrap ClassLoader)中；
- 只有当父级加载器反馈无法完成特定类或资源的加载请求时，即父级加载器在它的搜索范围内找不到该类或资源时（搜索范围是磁盘上对应的目录），子级加载器才尝试在自己的搜索范围内加载类或资源；
- 通过双亲委派模型，JVM 确保了：类的唯一性 uniqueness, 类的可见性 Visibility, 类的安全性 Security;
- 双亲委派模型也有不足，比如 java.sql.DriverManager/java.sq.Driver 等是 jre 核心内部类，在 $JAVA_HOME/jre/lib/rt.jar 中，由 bootstrap classloader 负责加载他们；而 jdbc 实现类如 oracle.jdbc.driver.OracleDriver 是在程序运行时指定的类加载路径下的某个 jar 包中（类加载路径是通过环境变量 classpath 动态指定的），由 application class loader 负责加载他们；所以当 bootstrap classloader 加载的 java.sql.DriverManager， 需要调用 application classloader 加载的 java.sq.Driver 的具体实现类 oracle.jdbc.driver.OracleDriver 时，由于类的可见性 （子类加载器加载的类对父类加载器加载的类默认不可见），按照双亲委派模型，这是不 work 的；
- 双亲委派模型的不足, 可以通过使用 thread Context Classloaders 来显示加载需要加载的类来弥补，JDBC/JNDI 等的实现，都使用了thread Context Classloaders；比如方法 java.sql.DriverManager#getConnection(java.lang.String, java.util.Properties, java.lang.Class<?>) 就借用了 thread context classloader， 来加载 jdbc 实现类如 oracle.jdbc.driver.OracleDriver，具体源码可见：
callerCL = Thread.currentThread().getContextClassLoader(); // 获得thread context classloader aClass = Class.forName(driver.getClass().getName(), true, classLoader); // // 使用获得的 thread context classloader，通过 Class.forName 加载 jdbc 实现类如 oracle.jdbc.driver.OracleDriver
- 当 JVM 内置的加载器满足不了需求时，可以自定义类加载器, 自定义类加载器只需要继承类 ClassLoader 并复写方法 findClass() 即可；
- 除了由 JRE 隐式动态加载类，还可以在代码中显示加载类；
- 显示加载类，可以通过 Class.forName(String name, boolean initialize,ClassLoader loader) 或 Class.forName(String name) 加载，其中后者默认会使用 system classloader，且默认会初始化类 （即执行类的静态代码块）；
- 显示加载类，也可以通过 classLoader.loadClass(String name) 来加载类，此时只会加载类但不会初始化类（即不会执行类的静态代码块）;
- 通过 classLoader.loadClass(String name) 来显示加载类时，类加载器可以是当前类加载器/系统类加载器/线程上下文类加载器/或新创建的URLClassLoader：
- 通过当前类加载器加载：Class.forName("com.hundsun.SimpleApp",true,urlContextClassLoader);
- 通过系统类加载器加载：ClassLoader.getSystemClassLoader().loadClass("com.hundsun.SimpleApp");
- 通过线程上下文类加载器加载：Thread.currentThread().getContextClassLoader().loadClass("com.hundsun.SimpleApp");
- 新建URLClassLoader并进行加载: ClassLoader prevContextClassLoader = Thread.currentThread().getContextClassLoader(); URLClassLoader urlContextClassLoader = new URLClassLoader(urls, prevContextClassLoader)；urlContextClassLoader.loadClass("com.hundsun.SimpleApp");
- 新建URLClassLoader时可以指定其父类，当不指定时其父类默认是系统类加载器加载；
