所属文集:ClassLoader串烧
同一个命名空间中,类只加载一份
AppClassLoader加载程序中自定义的类。无论加载多少次,只要是被AppClassLoader加载的,其Class信息hashcode都是相同的。
子加载器可见父加载器加载的类
到处都是例子。比如核心类库的类,AppClassLoader所加载的类,都能使用String
父加载器不可见子加载器所加载的类,只能验证加载不到
应用中有Parent类,Son类。Parent类中有个getSon方法中,使用Son类。
编译后,在classpath下,移除Son类到子加载器的加载目录中,让AppClassLoader无法加载。
达到我们的运行环境要求:Parent由父加载器加载,Son由子加载器加载.
调用Parent的getSon方法的时候要new Son类,会触发Son类的加载,但是加载不到Son类,报错信息为:java.lang.NoClassDefFoundError: com/rock/Son
public class Parent {
private Object son;
public Object getSon(){
return new Son();
}
public Object setSon(Object son){
this.son = son;
return this.son;
}
}
public class Son {
}
/**
* 父加载器加载不了,子加载器所加载的类,
* 父加载器加载Parent
* 子加载器加载son
* Parent#getSon 方法里 new Son()对象.//报错,找不到Son的类定义.
*/
@Test
public void testParentCanntFindSon(){
CustomClassLoader01 customClassLoader01 = new CustomClassLoader01(ClassLoader.getSystemClassLoader());
try {
Class> classParent = customClassLoader01.loadClass("com.rock.Parent");
System.out.println("classParent:" + classParent.getClassLoader());
Class> classSon = customClassLoader01.loadClass("com.rock.Son");
System.out.println("classSon:" + classSon.getClassLoader());
Object objParent = classParent.newInstance();
Object objSon = classSon.newInstance();
Method setSon = classParent.getMethod("setSon",Object.class);
Object resultSon = setSon.invoke(objParent, objSon);
System.out.println(resultSon.getClass());
System.out.println(resultSon.getClass().getClassLoader());
try {
Method getSon = classParent.getMethod("getSon");
Object invoke = getSon.invoke(objParent);
System.out.println(invoke.getClass().getClassLoader());
}catch (Exception exp){
//java.lang.NoClassDefFoundError: com/rock/Son
exp.printStackTrace();
}
}catch (Exception exp){
exp.printStackTrace();
}
}
不同命名空间的互相不可见
自定义类加载器的多个实例,分别加载Man类,并反射实例化对象,对象之间互相调用setFather赋值,会报错。a空间的不识别b空间的,即使全限定名相同;但也不识别。
public class Man {
private Man father;
public void setFather(Object obj){
father = (Man)obj;
}
}
@Test
public void testSifferentNamespaceClass(){
CustomClassLoader01 customClassLoader01 = new CustomClassLoader01(ClassLoader.getSystemClassLoader());
CustomClassLoader01 customClassLoader02 = new CustomClassLoader01(ClassLoader.getSystemClassLoader());
try {
Class> aClass1 = customClassLoader01.loadClass("com.rock.Man");
System.out.println("class1 : " + aClass1.getClassLoader());
System.out.println("class1 : " + aClass1.);
Class> aClass2 = customClassLoader02.loadClass("com.rock.Man");
System.out.println("class2 : " + aClass1.getClassLoader());
System.out.println("class2 : " + aClass2);
Object man1 = aClass1.newInstance();
Object man2 = aClass2.newInstance();
Method setFather = aClass1.getMethod("setFather", Object.class);
setFather.invoke(man1,man2);
}catch (Exception exp){
exp.printStackTrace();
}
}
class1 : com.rock.classLoader.CustomClassLoader01@1f28c152
class1 : 2006034581
class2 : com.rock.classLoader.CustomClassLoader01@1f28c152
class2 : 488044861
...
Caused by: java.lang.ClassCastException: com.rock.Man cannot be cast to com.rock.Man
at com.rock.Man.setFather(Man.java:6)
... 27 more
命名空间的设计实现
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