类加载器
java类加载器就是在运行时在jvm中动态地加载所需的类,java类加载器基于三个机制:委托,可见,单一。
把classpath下的那些.class文件加载进内存,处理后成为字节码,这些工作是类加载器做的。
默认类加载器
系统默认三个类加载器:
类加载器也是java类,而bootstrap不是。 验证代码:
public class classloadertest { public static void main(string[] args) { system.out.println(system.class.getclassloader()); } }
输出:null
如果使用system.out.println(system.class.getclassloader().tostring);,则报空指针异常:
exception in thread "main" java.lang.nullpointerexception at com.iot.classloader.classloadertest.main(classloadertest.java:10) at sun.reflect.nativemethodaccessorimpl.invoke0(native method) at sun.reflect.nativemethodaccessorimpl.invoke(nativemethodaccessorimpl.java:62) at sun.reflect.delegatingmethodaccessorimpl.invoke(delegatingmethodaccessorimpl.java:43) at java.lang.reflect.method.invoke(method.java:483) at com.intellij.rt.execution.application.appmain.main(appmain.java:144)
可见,system类是由bootstrap类加载器加载。
类加载器的委托机制
类加载器的树状图
类加载器
一般加载类的顺序:
自定义类加载器的编写原理
api:
class classloader
模板方法设计模式
父类:
loadclass(类加载的流程,模板)
findclass供子类覆盖的、被loadclass方法调用的类加载逻辑
defineclass得到class文件转换成字节码
子类:覆盖findclass方法
例子:
loadclass方法的源码
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; } }
api文档中的例子:
class networkclassloader extends classloader { string host; int port; public class findclass(string name) { byte[] b = loadclassdata(name); return defineclass(name, b, 0, b.length); } private byte[] loadclassdata(string name) { // load the class data from the connection . . . } }
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