Java类加载器

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类加载器，ClassLoader，用于完成类的加载工作。对于一个类来说，它在VM中的唯一性是由该类本身和类加载器一起决定的。

双亲委派模型

如果一个类加载器收到一个类加载的请求，它首先不会自己尝试加载这个类，而是交给它的父类加载器去完成，父加载器又交给父父类加载器直到顶层类加载器；只有当父类加载器无法完成这个类加载请求时才会将其交给子类去做。

protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {    synchronized (getClassLoadingLock(name)) {        // First, check if the class has already been loaded        Class<?> c = findLoadedClass(name);        if (c == null) {            long t0 = System.nanoTime();            try {                if (parent != null) {                    c = parent.loadClass(name, false);                } else {                    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();                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;    }}

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