标签:委派 scl Launcher 双亲 oops catch 机制 null 加载
一.类加载器分为四类:
1.启动类加载器(Bootstrap classLoader):负责加载支撑JVM运行的位于JRE的lib目录下的核心类库,比如 rt.jar、charsets.jar等
2.扩展类加载器(ExtClassLoader):负责加载支撑JVM运行的位于JRE的lib目录下的ext扩展目录中 的JAR类包
3.应用程序类加载器(AppClassLoader):负责加载ClassPath路径下的类包,主要就是加载你自己写(即项目对应路径下的) 的那些类
4.自定义加载器:负责加载用户自定义路径下的类包
二.加载机制介绍:(双亲委派)
双亲委派机制整体流程图示:
源码分析:
protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException { synchronized (getClassLoadingLock(name)) { // First, check if the class has already been loaded //判断该类是否已经被加载 Class<?> c = findLoadedClass(name); if (c == null) { long t0 = System.nanoTime(); try { if (parent != null) { //父类不为空交给父类去加载 c = parent.loadClass(name, false); } else { //父类为空则说明,说明是启动类加载器, //因为底层是c说编写,java是找不到该类的 c = findBootstrapClassOrNull(name); } } catch (ClassNotFoundException e) { // ClassNotFoundException thrown if class not found // from the non-null parent class loader } if (c == null) { // If still not found, then invoke findClass in order // to find the class. long t1 = System.nanoTime(); //如果加载不到,怎调用其子类的方法去找 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) { resolveClass(c); } return c; } }
其中 ext和app都是classLoader的子类不存在真实的继承关系,都是Launcher的内部类,通过Launcher类的初始的时候设置伪父类的关系:
public Launcher() { Launcher.ExtClassLoader var1; try { var1 = Launcher.ExtClassLoader.getExtClassLoader(); } catch (IOException var10) { throw new InternalError("Could not create extension class loader", var10); } try { this.loader = Launcher.AppClassLoader.getAppClassLoader(var1); } catch (IOException var9) { throw new InternalError("Could not create application class loader", var9); } Thread.currentThread().setContextClassLoader(this.loader); String var2 = System.getProperty("java.security.manager"); if (var2 != null) { SecurityManager var3 = null; if (!"".equals(var2) && !"default".equals(var2)) { try { var3 = (SecurityManager)this.loader.loadClass(var2).newInstance(); } catch (IllegalAccessException var5) { } catch (InstantiationException var6) { } catch (ClassNotFoundException var7) { } catch (ClassCastException var8) { } } else { var3 = new SecurityManager(); } if (var3 == null) { throw new InternalError("Could not create SecurityManager: " + var2); } System.setSecurityManager(var3); } }
创建这两个加载器的时候都会去调用classLoader的构造器,将其父类设值
protected ClassLoader(ClassLoader parent) { this(checkCreateClassLoader(), parent); }
当我们自定义类加载器的时候,只需要重写其findClass方法即可,如果要打破双亲委派机制,重新loadClass方法,其默认的构造器是将appClassLoader。
protected ClassLoader() { this(checkCreateClassLoader(), getSystemClassLoader()); }
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; } }
标签:委派,scl,Launcher,双亲,oops,catch,机制,null,加载 来源: https://blog.csdn.net/cdtu007/article/details/120888359
本站声明: 1. iCode9 技术分享网(下文简称本站)提供的所有内容,仅供技术学习、探讨和分享; 2. 关于本站的所有留言、评论、转载及引用,纯属内容发起人的个人观点,与本站观点和立场无关; 3. 关于本站的所有言论和文字,纯属内容发起人的个人观点,与本站观点和立场无关; 4. 本站文章均是网友提供,不完全保证技术分享内容的完整性、准确性、时效性、风险性和版权归属;如您发现该文章侵犯了您的权益,可联系我们第一时间进行删除; 5. 本站为非盈利性的个人网站,所有内容不会用来进行牟利,也不会利用任何形式的广告来间接获益,纯粹是为了广大技术爱好者提供技术内容和技术思想的分享性交流网站。