我想使用 Class.newInstance()
,但是我要实例化的类没有无参构造函数。因此,我需要能够传入构造函数参数。是否有一种方法可以做到这一点?
我想使用 Class.newInstance()
,但是我要实例化的类没有无参构造函数。因此,我需要能够传入构造函数参数。是否有一种方法可以做到这一点?
MyClass.class.getDeclaredConstructor(String.class).newInstance("HERESMYARG");
或者obj.getClass().getDeclaredConstructor(String.class).newInstance("HERESMYARG");
getDeclaredConstructor
不是静态方法,你需要在特定类的实例上调用它。 - clumclass MyClass {
public MyClass(Long l, String s, int i) {
}
}
您需要展示您打算像这样使用该构造函数:
Class classToLoad = MyClass.class;
Class[] cArg = new Class[3]; //Our constructor has 3 arguments
cArg[0] = Long.class; //First argument is of *object* type Long
cArg[1] = String.class; //Second argument is of *object* type String
cArg[2] = int.class; //Third argument is of *primitive* type int
Long l = new Long(88);
String s = "text";
int i = 5;
classToLoad.getDeclaredConstructor(cArg).newInstance(l, s, i);
按照以下步骤调用带参数的构造函数:
Class[]
中传递类型来获取具有参数类型的构造函数,使用 Class
的 getDeclaredConstructor
方法Object[]
中传递值来创建构造函数实例,使用 Constructor
的 newInstance
方法示例代码:
import java.lang.reflect.*;
class NewInstanceWithReflection{
public NewInstanceWithReflection(){
System.out.println("Default constructor");
}
public NewInstanceWithReflection( String a){
System.out.println("Constructor :String => "+a);
}
public static void main(String args[]) throws Exception {
NewInstanceWithReflection object = (NewInstanceWithReflection)Class.forName("NewInstanceWithReflection").newInstance();
Constructor constructor = NewInstanceWithReflection.class.getDeclaredConstructor( new Class[] {String.class});
NewInstanceWithReflection object1 = (NewInstanceWithReflection)constructor.newInstance(new Object[]{"StackOverFlow"});
}
}
输出:
java NewInstanceWithReflection
Default constructor
Constructor :String => StackOverFlow
getDeclaredConstructor 方法。它期望一个类数组。这是一个经过测试和工作的示例:
public static JFrame createJFrame(Class c, String name, Component parentComponent)
{
try
{
JFrame frame = (JFrame)c.getDeclaredConstructor(new Class[] {String.class}).newInstance("name");
if (parentComponent != null)
{
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
else
{
frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
}
frame.setLocationRelativeTo(parentComponent);
frame.pack();
frame.setVisible(true);
}
catch (InstantiationException instantiationException)
{
ExceptionHandler.handleException(instantiationException, parentComponent, Language.messages.get(Language.InstantiationExceptionKey), c.getName());
}
catch(NoSuchMethodException noSuchMethodException)
{
//ExceptionHandler.handleException(noSuchMethodException, parentComponent, Language.NoSuchMethodExceptionKey, "NamedConstructor");
ExceptionHandler.handleException(noSuchMethodException, parentComponent, Language.messages.get(Language.NoSuchMethodExceptionKey), "(Constructor or a JFrame method)");
}
catch (IllegalAccessException illegalAccessException)
{
ExceptionHandler.handleException(illegalAccessException, parentComponent, Language.messages.get(Language.IllegalAccessExceptionKey));
}
catch (InvocationTargetException invocationTargetException)
{
ExceptionHandler.handleException(invocationTargetException, parentComponent, Language.messages.get(Language.InvocationTargetExceptionKey));
}
finally
{
return null;
}
}
这是我如何使用动态构造函数参数列表创建 Class clazz
实例的方法。
final Constructor constructor = clazz.getConstructors()[0];
final int constructorArgsCount = constructor.getParameterCount();
if (constructorArgsCount > 0) {
final Object[] constructorArgs = new Object[constructorArgsCount];
int i = 0;
for (Class parameterClass : constructor.getParameterTypes()) {
Object dummyParameterValue = getDummyValue(Class.forName(parameterClass.getTypeName()), null);
constructorArgs[i++] = dummyParameterValue;
}
instance = constructor.newInstance(constructorArgs);
} else {
instance = clazz.newInstance();
}
这是 getDummyValue()
方法的代码实现:
private static Object getDummyValue(final Class clazz, final Field field) throws Exception {
if (int.class.equals(clazz) || Integer.class.equals(clazz)) {
return DUMMY_INT;
} else if (String.class.equals(clazz)) {
return DUMMY_STRING;
} else if (boolean.class.equals(clazz) || Boolean.class.equals(clazz)) {
return DUMMY_BOOL;
} else if (List.class.equals(clazz)) {
Class fieldClassGeneric = Class.forName(((ParameterizedType) field.getGenericType()).getActualTypeArguments()[0].getTypeName());
return List.of(getDummyValue(fieldClassGeneric, null));
} else if (USER_DEFINED_CLASSES.contains(clazz.getSimpleName())) {
return createClassInstance(clazz);
} else {
throw new Exception("Dummy value for class type not defined - " + clazz.getName();
}
}