【原创】Java基础之ClassLoader类加载器简介

classloader简介

1 classloader层次结构（父子关系）

Bootstrap（ClassLoader）

• ExtClassLoader
  • AppClassLoader
    • XXXClassLoader

详见代码如下：

sun.misc.Launcher

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);
 
        }

2 classloader加载类过程

classloader在加载class时优先尝试通过parent classloader加载，如果加载不到，才尝试自己加载，即常说的“双亲委派”，

并且如果一个class依赖其他class，比如ClassA依赖ClassB，ClassB默认由ClassA的classloader来加载。

* <p> The methods and constructors of objects created by a class loader may
* reference other classes. To determine the class(es) referred to, the Java
* virtual machine invokes the {@link #loadClass <tt>loadClass</tt>} method of
* the class loader that originally created the class.

详见代码如下：

java.lang.ClassLoader

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;
 
        }
 
    }