只有当类具有默认的构造器时，才能调用Class.newInstance()；否则抛异常：具体执行情况可以调试进去看看。
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at java.lang.Class.newInstance0(Class.java:340)
at java.lang.Class.newInstance(Class.java:308)
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下面是关键代码调用:
public T newInstance() throws InstantiationException, IllegalAccessException
{
if (System.getSecurityManager() != null) {
checkMemberAccess(Member.PUBLIC, ClassLoader.getCallerClassLoader());
}
return newInstance0();
}
private T newInstance0() throws InstantiationException, IllegalAccessException
{
............
try {
Class[] empty = {};
final Constructor<T> c = getConstructor0(empty, Member.DECLARED);
................
}catch (NoSuchMethodException e) {
throw new InstantiationException(getName());
}
}
private Constructor<T> getConstructor0(Class[] parameterTypes, int which) throws NoSuchMethodException
{
Constructor[] constructors = privateGetDeclaredConstructors((which == Member.PUBLIC));
for (int i = 0; i < constructors.length; i++) {
if (arrayContentsEq(parameterTypes, constructors[i].getParameterTypes())) {
return getReflectionFactory().copyConstructor(constructors[i]);
}
}
throw new NoSuchMethodException(getName() + ".<init>" + argumentTypesToString(parameterTypes));
}