public class Smile { static { System.out.println("smile static"); } public Smile() { System.out.println("smile init"); }}public class Lxy { static { System.out.println("lxy static"); } public Lxy() { System.out.println("lxy init"); }}public class ClassLoaderTest public static void main(String[] args) { new Smile(); System.out.println("---------------"); new Lxy(); }}
jvm 中添加
-verbose:class 或者 -XX:+TraceClassLoading
轨迹展示
[Opened /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar][Loaded java.lang.Object from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar][Loaded java.io.Serializable from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar][Loaded java.lang.Comparable from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar]...[Loaded java.lang.Cloneable from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar][Loaded java.lang.ClassLoader from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar]...[Loaded java.net.URLClassLoader from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar][Loaded java.net.URL from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar][Loaded java.util.jar.Manifest from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar][Loaded sun.misc.Launcher from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar][Loaded sun.misc.Launcher$AppClassLoader from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar][Loaded sun.misc.Launcher$ExtClassLoader from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar]...[Loaded sun.launcher.LauncherHelper$FXHelper from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar][Loaded java.lang.Class$MethodArray from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar][Loaded java.net.AbstractPlainSocketImpl$1 from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar][Loaded java.lang.Void from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar][Loaded com.smile.Smile from file:/Users/smile/workSpace/resource/git/temp/smile-company/target/test-classes/]smile staticsmile init---------------[Loaded java.net.Inet6Address from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar][Loaded com.smile.Lxy from file:/Users/smile/workSpace/resource/git/temp/smile-company/target/test-classes/]lxy staticlxy init[Loaded java.lang.Shutdown from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar][Loaded java.lang.Shutdown$Lock from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar][Loaded java.net.Inet6Address$Inet6AddressHolder from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar]
public abstract class ClassLoader { public Class<?> loadClass(String name) throws ClassNotFoundException { return loadClass(name, false); } protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException { synchronized (getClassLoadingLock(name)) { // 1. 查看是否已加载 Class<?> c = findLoadedClass(name); if (c == null) { // 未加载 long t0 = System.nanoTime(); try { if (parent != null) { // 若父类不为null, 委托父类加载 c = parent.loadClass(name, false); } else { // 父类为null, 委托Bootstrap加载(JNI) c = findBootstrapClassOrNull(name); } } catch (ClassNotFoundException e) { // ClassNotFoundException thrown if class not found // from the non-null parent class loader } if (c == null) { // 父类中不存在, 从当前加载器中寻找 // If still not found, then invoke findClass in order // to find the class. long t1 = System.nanoTime(); c = findClass(name); // this is the defining class loader; record the stats sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0); sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1); sun.misc.PerfCounter.getFindClasses().increment(); } } if (resolve) { // 是否解析 resolveClass(c); } return c; } }}
public class Launcher { static class AppClassLoader extends URLClassLoader { public Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException { int i = name.lastIndexOf('.'); if (i != -1) { SecurityManager sm = System.getSecurityManager(); if (sm != null) { sm.checkPackageAccess(name.substring(0, i)); // 检测文件路径是否允许访问 } } return (super.loadClass(name, resolve)); } } }
public abstract class ClassLoader { private final ClassLoader parent; protected ClassLoader() { this(checkCreateClassLoader(), getSystemClassLoader()); } private ClassLoader(Void unused, ClassLoader parent) { this.parent = parent; if (ParallelLoaders.isRegistered(this.getClass())) { parallelLockMap = new ConcurrentHashMap<>(); package2certs = new ConcurrentHashMap<>(); domains = Collections.synchronizedSet(new HashSet<ProtectionDomain>()); assertionLock = new Object(); } else { // no finer-grained lock; lock on the classloader instance parallelLockMap = null; package2certs = new Hashtable<>(); domains = new HashSet<>(); assertionLock = this; } } @CallerSensitive public static ClassLoader getSystemClassLoader() { initSystemClassLoader(); if (scl == null) { return null; } SecurityManager sm = System.getSecurityManager(); if (sm != null) { checkClassLoaderPermission(scl, Reflection.getCallerClass()); } return scl; } private static synchronized void initSystemClassLoader() { if (!sclSet) { if (scl != null) throw new IllegalStateException("recursive invocation"); sun.misc.Launcher l = sun.misc.Launcher.getLauncher(); if (l != null) { Throwable oops = null; scl = l.getClassLoader(); try { scl = AccessController.doPrivileged( new SystemClassLoaderAction(scl)); } catch (PrivilegedActionException pae) { oops = pae.getCause(); if (oops instanceof InvocationTargetException) { oops = oops.getCause(); } } if (oops != null) { if (oops instanceof Error) { throw (Error) oops; } else { // wrap the exception throw new Error(oops); } } } sclSet = true; } } }
从上所得, BootStrap 是在 ClassLoader 初始化时初始化
public abstract class ClassLoader { private Class<?> findBootstrapClassOrNull(String name) { if (!checkName(name)) return null; return findBootstrapClass(name); } private native Class<?> findBootstrapClass(String name);}
发现findBootstrapClass标有native, 底层实现使用 C 语言实现的, 所有无法看到 java 的实现
若委托父类找不到, 则尝试从当前类加载器中 findClass(name) 加载
public class URLClassLoader extends SecureClassLoader implements Closeable { private final URLClassPath ucp; protected Class<?> findClass(final String name) throws ClassNotFoundException { try { return AccessController.doPrivileged( new PrivilegedExceptionAction<Class<?>>() { public Class<?> run() throws ClassNotFoundException { String path = name.replace('.', '/').concat(".class"); // 拼装文件路径 Resource res = ucp.getResource(path, false); // 从当前类加载器负责加载的路径中获取资源 if (res != null) { try { return defineClass(name, res); // 将资源转换为Class } catch (IOException e) { throw new ClassNotFoundException(name, e); } } else { throw new ClassNotFoundException(name); } } }, acc); } catch (java.security.PrivilegedActionException pae) { throw (ClassNotFoundException) pae.getException(); } } private Class<?> defineClass(String name, Resource res) throws IOException { long t0 = System.nanoTime(); int i = name.lastIndexOf('.'); URL url = res.getCodeSourceURL(); if (i != -1) { String pkgname = name.substring(0, i); // 检测包是否已加载 Manifest man = res.getManifest(); // 编译后类的相关属性(编译版本号...) if (getAndVerifyPackage(pkgname, man, url) == null) { // 未加载 try { if (man != null) { definePackage(pkgname, man, url); // 定义包 } else { definePackage(pkgname, null, null, null, null, null, null, null); } } catch (IllegalArgumentException iae) { // parallel-capable class loaders: re-verify in case of a // race condition if (getAndVerifyPackage(pkgname, man, url) == null) { // Should never happen throw new AssertionError("Cannot find package " + pkgname); } } } } // Now read the class bytes and define the class java.nio.ByteBuffer bb = res.getByteBuffer(); // 转换为字节 if (bb != null) { // Use (direct) ByteBuffer: CodeSigner[] signers = res.getCodeSigners(); // 资源签名 CodeSource cs = new CodeSource(url, signers); // 代码源 sun.misc.PerfCounter.getReadClassBytesTime().addElapsedTimeFrom(t0); // 用来监控加载该资源消耗时间 return defineClass(name, bb, cs); // 转换为Class } else { byte[] b = res.getBytes(); // must read certificates AFTER reading bytes. CodeSigner[] signers = res.getCodeSigners(); CodeSource cs = new CodeSource(url, signers); sun.misc.PerfCounter.getReadClassBytesTime().addElapsedTimeFrom(t0); return defineClass(name, b, 0, b.length, cs); } } } public class URLClassPath { public Resource getResource(String name, boolean check) { if (DEBUG) { System.err.println("URLClassPath.getResource(\"" + name + "\")"); } Loader loader; for (int i = 0; (loader = getLoader(i)) != null; i++) { Resource res = loader.getResource(name, check); if (res != null) { return res; } } return null; } private synchronized Loader getLoader(int index) { if (closed) { return null; } // Expand URL search path until the request can be satisfied // or the URL stack is empty. while (loaders.size() < index + 1) { // Pop the next URL from the URL stack URL url; synchronized (urls) { if (urls.empty()) { return null; } else { url = urls.pop(); } } // Skip this URL if it already has a Loader. (Loader // may be null in the case where URL has not been opened // but is referenced by a JAR index.) String urlNoFragString = URLUtil.urlNoFragString(url); if (lmap.containsKey(urlNoFragString)) { continue; } // Otherwise, create a new Loader for the URL. Loader loader; try { loader = getLoader(url); // If the loader defines a local class path then add the // URLs to the list of URLs to be opened. URL[] urls = loader.getClassPath(); if (urls != null) { push(urls); } } catch (IOException e) { // Silently ignore for now... continue; } // Finally, add the Loader to the search path. loaders.add(loader); lmap.put(urlNoFragString, loader); } return loaders.get(index); } /* * Returns the Loader for the specified base URL. */ private Loader getLoader(final URL url) throws IOException { try { return java.security.AccessController.doPrivileged( new java.security.PrivilegedExceptionAction<Loader>() { public Loader run() throws IOException { String file = url.getFile(); if (file != null && file.endsWith("/")) { if ("file".equals(url.getProtocol())) { return new FileLoader(url); } else { return new Loader(url); } } else { return new JarLoader(url, jarHandler, lmap); } } }); } catch (java.security.PrivilegedActionException pae) { throw (IOException)pae.getException(); } } static class JarLoader extends Loader { Resource getResource(final String name, boolean check) { if (metaIndex != null) { if (!metaIndex.mayContain(name)) { return null; } } try { ensureOpen(); } catch (IOException e) { throw new InternalError(e); } final JarEntry entry = jar.getJarEntry(name); if (entry != null) return checkResource(name, check, entry); if (index == null) return null; HashSet<String> visited = new HashSet<String>(); return getResource(name, check, visited); } private void ensureOpen() throws IOException { if (jar == null) { try { java.security.AccessController.doPrivileged( new java.security.PrivilegedExceptionAction<Void>() { public Void run() throws IOException { if (DEBUG) { System.err.println("Opening " + csu); Thread.dumpStack(); } jar = getJarFile(csu); index = JarIndex.getJarIndex(jar, metaIndex); if (index != null) { String[] jarfiles = index.getJarFiles(); // Add all the dependent URLs to the lmap so that loaders // will not be created for them by URLClassPath.getLoader(int) // if the same URL occurs later on the main class path. We set // Loader to null here to avoid creating a Loader for each // URL until we actually need to try to load something from them. for(int i = 0; i < jarfiles.length; i++) { try { URL jarURL = new URL(csu, jarfiles[i]); // If a non-null loader already exists, leave it alone. String urlNoFragString = URLUtil.urlNoFragString(jarURL); if (!lmap.containsKey(urlNoFragString)) { lmap.put(urlNoFragString, null); } } catch (MalformedURLException e) { continue; } } } return null; } } ); } catch (java.security.PrivilegedActionException pae) { throw (IOException)pae.getException(); } } } .... } } public class SecureClassLoader extends ClassLoader { protected final Class<?> defineClass(String name, byte[] b, int off, int len, CodeSource cs) { return defineClass(name, b, off, len, getProtectionDomain(cs)); } protected final Class<?> defineClass(String name, byte[] b, int off, int len, ProtectionDomain protectionDomain) throws ClassFormatError { protectionDomain = preDefineClass(name, protectionDomain); String source = defineClassSourceLocation(protectionDomain); Class<?> c = defineClass1(name, b, off, len, protectionDomain, source); postDefineClass(c, protectionDomain); return c; } }
public class JdbcClassLoaderTest { public static void main(String[] args) { Enumeration<Driver> drivers = DriverManager.getDrivers(); while (drivers.hasMoreElements()) { Driver driver = drivers.nextElement(); System.out.println(String.format("driver: %s, classLoader: %s", driver.getClass(), driver.getClass().getClassLoader().getClass())); } System.err.println(String.format("driver: %s, classLoader: %s", DriverManager.class, Objects.nonNull(DriverManager.class.getClassLoader()) ? DriverManager.class.getClassLoader().getClass() : DriverManager.class.getClassLoader())); }}
结果展示
driver: class com.mysql.jdbc.Driver, classLoader: class sun.misc.Launcher$AppClassLoaderdriver: class com.mysql.fabric.jdbc.FabricMySQLDriver, classLoader: class sun.misc.Launcher$AppClassLoaderdriver: class com.alibaba.druid.proxy.DruidDriver, classLoader: class sun.misc.Launcher$AppClassLoaderdriver: class com.alibaba.druid.mock.MockDriver, classLoader: class sun.misc.Launcher$AppClassLoaderdriver: class java.sql.DriverManager, classLoader: null
分析
public class DriverManager { private final static CopyOnWriteArrayList<DriverInfo> registeredDrivers = new CopyOnWriteArrayList<>(); static { loadInitialDrivers(); // 1. 初始化Drivers println("JDBC DriverManager initialized"); } private static void loadInitialDrivers() { String drivers; // 2. Jvm启动时指定驱动实现(多个驱动可用':'分隔), 将在下面采用BootStrapClassLoader加载. try { drivers = AccessController.doPrivileged(new PrivilegedAction<String>() { public String run() { return System.getProperty("jdbc.drivers"); } }); } catch (Exception ex) { drivers = null; } // 通过SPI机制加载(ServiceLoader.load(Driver.class)) AccessController.doPrivileged(new PrivilegedAction<Void>() { public Void run() { ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class); // 3. SPI获取各厂商实现(懒加载) Iterator<Driver> driversIterator = loadedDrivers.iterator(); // 4. 转换为Iterator /* Load these drivers, so that they can be instantiated. * It may be the case that the driver class may not be there * i.e. there may be a packaged driver with the service class * as implementation of java.sql.Driver but the actual class * may be missing. In that case a java.util.ServiceConfigurationError * will be thrown at runtime by the VM trying to locate * and load the service. * * Adding a try catch block to catch those runtime errors * if driver not available in classpath but it's * packaged as service and that service is there in classpath. */ try{ while(driversIterator.hasNext()) { // 5. 是否有元素(执行SPI解析) driversIterator.next(); // 6. 加载文件(Class.forName) } } catch(Throwable t) { // Do nothing } return null; } }); println("DriverManager.initialize: jdbc.drivers = " + drivers); if (drivers == null || drivers.equals("")) { return; } String[] driversList = drivers.split(":"); println("number of Drivers:" + driversList.length); for (String aDriver : driversList) { try { println("DriverManager.Initialize: loading " + aDriver); Class.forName(aDriver, true, ClassLoader.getSystemClassLoader()); } catch (Exception ex) { println("DriverManager.Initialize: load failed: " + ex); } } }}public final class ServiceLoader<S> implements Iterable<S> { private static final String PREFIX = "META-INF/services/"; // The class or interface representing the service being loaded private final Class<S> service; // 用于加载对象的类加载器 private final ClassLoader loader; // 加载对象时的权限上下文 private final AccessControlContext acc; // 缓存 private LinkedHashMap<String,S> providers = new LinkedHashMap<>(); // 具体实现的迭代器(懒迭代) private LazyIterator lookupIterator; /** * 使用当前线程类加载器加载具体实现 */ public static <S> ServiceLoader<S> load(Class<S> service) { ClassLoader cl = Thread.currentThread().getContextClassLoader(); // 3.1 获取当前线程类加载器 return ServiceLoader.load(service, cl); // 3.2 使用当前线程类加载读取具体实现(这里就是**破坏双亲委托机制**的地方) } public static <S> ServiceLoader<S> load(Class<S> service, ClassLoader loader) { return new ServiceLoader<>(service, loader); // 3.2.1 创建对象 } private ServiceLoader(Class<S> svc, ClassLoader cl) { service = Objects.requireNonNull(svc, "Service interface cannot be null"); loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl; // 3.2.1.1 若没有提供类加载器, 则使用BootStrapClassLoader类加载器 acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null; // 3.2.1.2 权限上下文 reload(); // 3.2.1.3 刷新缓存, 新建迭代器 } public void reload() { providers.clear(); // 3.2.1.3.1 刷新缓存 lookupIterator = new LazyIterator(service, loader); // 3.2.1.3.2 新建迭代器 } /** * 封装迭代器 */ public Iterator<S> iterator() { return new Iterator<S>() { // 4.1 转换为迭代器 Iterator<Map.Entry<String,S>> knownProviders = providers.entrySet().iterator(); public boolean hasNext() { if (knownProviders.hasNext()) // 5.1 已知提供者是否存在元素 return true; return lookupIterator.hasNext(); // 5.2 懒迭代中是否存在 } public S next() { if (knownProviders.hasNext()) // 6.1 已知提供者是否存在元素 return knownProviders.next().getValue(); return lookupIterator.next(); // 6.2 懒迭代中获取下一个元素 } public void remove() { throw new UnsupportedOperationException(); } }; } private class LazyIterator implements Iterator<S> { Class<S> service; ClassLoader loader; Enumeration<URL> configs = null; Iterator<String> pending = null; String nextName = null; private LazyIterator(Class<S> service, ClassLoader loader) { this.service = service; // 3.2.1.3.2.1 将要加载的接口 this.loader = loader; // 3.2.1.3.2.2 类加载器 } private boolean hasNextService() { if (nextName != null) { // 5.2.2.1 加载实现的接口名称 return true; } if (configs == null) { try { String fullName = PREFIX + service.getName(); // 5.2.2.2 SPI读取文件的路径 META-INF/services/java.sql.Driver if (loader == null) configs = ClassLoader.getSystemResources(fullName); else configs = loader.getResources(fullName); // 5.2.2.3 类加载器加载资源 ClassLoader#getResources } catch (IOException x) { fail(service, "Error locating configuration files", x); } } while ((pending == null) || !pending.hasNext()) { if (!configs.hasMoreElements()) { return false; } pending = parse(service, configs.nextElement()); // 5.2.2.4 解析URL元素获取实现名称s(com.mysql.jdbc.Driver, com.mysql.fabric.jdbc.FabricMySQLDriver) } nextName = pending.next(); // 5.2.2.5 下个元素名称 return true; } private S nextService() { if (!hasNextService()) // 6.2.2.1 是否存在具体实现 throw new NoSuchElementException(); String cn = nextName; // 6.2.2.2 (5.2.2.5 缓存的数据) nextName = null; Class<?> c = null; try { c = Class.forName(cn, false, loader); // 6.2.2.3 加载实现 } catch (ClassNotFoundException x) { fail(service, "Provider " + cn + " not found"); } if (!service.isAssignableFrom(c)) { // 6.2.2.4 SPI接口与 加载的类或超类是否相同 fail(service, "Provider " + cn + " not a subtype"); } try { S p = service.cast(c.newInstance()); // 6.2.2.5 强行转换为SPI接口 providers.put(cn, p); // 6.2.2.6 缓存 return p; } catch (Throwable x) { fail(service, "Provider " + cn + " could not be instantiated", x); } throw new Error(); // This cannot happen } public boolean hasNext() { if (acc == null) { // 5.2.1 权限上下文是否为null return hasNextService(); // 5.2.2 真正执行SPI解析 } else { PrivilegedAction<Boolean> action = new PrivilegedAction<Boolean>() { public Boolean run() { return hasNextService(); } }; return AccessController.doPrivileged(action, acc); } } public S next() { if (acc == null) { // 6.2.1 权限上下文是否为null return nextService(); // 6.2.2 类加载 } else { PrivilegedAction<S> action = new PrivilegedAction<S>() { public S run() { return nextService(); } }; return AccessController.doPrivileged(action, acc); } } public void remove() { throw new UnsupportedOperationException(); } }}
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