Troubleshooting
当try通过反射调用构造函数时,开发人员有时会遇到以下问题。
InstantiationException 由于缺少零参数构造函数
ConstructorTrouble示例说明了代码try使用Class.newInstance()创建类的新实例并且没有可访问的零参数构造函数时发生的情况:
public class ConstructorTrouble {
private ConstructorTrouble(int i) {}
public static void main(String... args){
try {
Class<?> c = Class.forName("ConstructorTrouble");
Object o = c.newInstance(); // InstantiationException
// production code should handle these exceptions more gracefully
} catch (ClassNotFoundException x) {
x.printStackTrace();
} catch (InstantiationException x) {
x.printStackTrace();
} catch (IllegalAccessException x) {
x.printStackTrace();
}
}
}
$ java ConstructorTrouble
java.lang.InstantiationException: ConstructorTrouble
at java.lang.Class.newInstance0(Class.java:340)
at java.lang.Class.newInstance(Class.java:308)
at ConstructorTrouble.main(ConstructorTrouble.java:7)
Tip:
出现InstantiationException的原因有多种。在这种情况下,问题在于带有int
参数的构造函数的存在会阻止编译器生成默认(或零参数)构造函数,并且代码中没有显式的零参数构造函数。请记住,Class.newInstance()的行为与new
关键字非常相似,并且一旦new
失败都会失败。
Class.newInstance()引发意外异常
ConstructorTroubleToo示例显示了Class.newInstance()中无法解决的问题。即,它传播构造函数引发的任何异常(已检查或未检查)。
import java.lang.reflect.InvocationTargetException;
import static java.lang.System.err;
public class ConstructorTroubleToo {
public ConstructorTroubleToo() {
throw new RuntimeException("exception in constructor");
}
public static void main(String... args) {
try {
Class<?> c = Class.forName("ConstructorTroubleToo");
// Method propagetes any exception thrown by the constructor
// (including checked exceptions).
if (args.length > 0 && args[0].equals("class")) {
Object o = c.newInstance();
} else {
Object o = c.getConstructor().newInstance();
}
// production code should handle these exceptions more gracefully
} catch (ClassNotFoundException x) {
x.printStackTrace();
} catch (InstantiationException x) {
x.printStackTrace();
} catch (IllegalAccessException x) {
x.printStackTrace();
} catch (NoSuchMethodException x) {
x.printStackTrace();
} catch (InvocationTargetException x) {
x.printStackTrace();
err.format("%n%nCaught exception: %s%n", x.getCause());
}
}
}
$ java ConstructorTroubleToo class
Exception in thread "main" java.lang.RuntimeException: exception in constructor
at ConstructorTroubleToo.<init>(ConstructorTroubleToo.java:6)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance
(NativeConstructorAccessorImpl.java:39)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance
(DelegatingConstructorAccessorImpl.java:27)
at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
at java.lang.Class.newInstance0(Class.java:355)
at java.lang.Class.newInstance(Class.java:308)
at ConstructorTroubleToo.main(ConstructorTroubleToo.java:15)
这种情况是反射所特有的。通常,不可能编写忽略已检查异常的代码,因为它不会编译。可以使用Constructor.newInstance()而不是Class.newInstance()来包装构造函数抛出的所有异常。
$ java ConstructorTroubleToo
java.lang.reflect.InvocationTargetException
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance
(NativeConstructorAccessorImpl.java:39)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance
(DelegatingConstructorAccessorImpl.java:27)
at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
at ConstructorTroubleToo.main(ConstructorTroubleToo.java:17)
Caused by: java.lang.RuntimeException: exception in constructor
at ConstructorTroubleToo.<init>(ConstructorTroubleToo.java:6)
... 5 more
Caught exception: java.lang.RuntimeException: exception in constructor
如果抛出InvocationTargetException,则调用该方法。问题的诊断与直接调用构造函数并引发由InvocationTargetException.getCause()检索的异常相同。此异常并不表示反射包或其用法有问题。
Tip:
最好使用Constructor.newInstance()而不是Class.newInstance(),因为以前的 API 允许检查和处理构造函数抛出的任意异常。
查找或调用正确的构造方法时出现问题
ConstructorTroubleAgain类说明了错误代码无法定位或调用预期构造函数的各种方式。
import java.lang.reflect.InvocationTargetException;
import static java.lang.System.out;
public class ConstructorTroubleAgain {
public ConstructorTroubleAgain() {}
public ConstructorTroubleAgain(Integer i) {}
public ConstructorTroubleAgain(Object o) {
out.format("Constructor passed Object%n");
}
public ConstructorTroubleAgain(String s) {
out.format("Constructor passed String%n");
}
public static void main(String... args){
String argType = (args.length == 0 ? "" : args[0]);
try {
Class<?> c = Class.forName("ConstructorTroubleAgain");
if ("".equals(argType)) {
// IllegalArgumentException: wrong number of arguments
Object o = c.getConstructor().newInstance("foo");
} else if ("int".equals(argType)) {
// NoSuchMethodException - looking for int, have Integer
Object o = c.getConstructor(int.class);
} else if ("Object".equals(argType)) {
// newInstance() does not perform method resolution
Object o = c.getConstructor(Object.class).newInstance("foo");
} else {
assert false;
}
// production code should handle these exceptions more gracefully
} catch (ClassNotFoundException x) {
x.printStackTrace();
} catch (NoSuchMethodException x) {
x.printStackTrace();
} catch (InvocationTargetException x) {
x.printStackTrace();
} catch (InstantiationException x) {
x.printStackTrace();
} catch (IllegalAccessException x) {
x.printStackTrace();
}
}
}
$ java ConstructorTroubleAgain
Exception in thread "main" java.lang.IllegalArgumentException: wrong number of
arguments
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance
(NativeConstructorAccessorImpl.java:39)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance
(DelegatingConstructorAccessorImpl.java:27)
at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
at ConstructorTroubleAgain.main(ConstructorTroubleAgain.java:23)
抛出IllegalArgumentException,是因为请求了零参数构造函数,并且试图传递参数。如果向构造函数传递了错误类型的参数,则将引发相同的异常。
$ java ConstructorTroubleAgain int
java.lang.NoSuchMethodException: ConstructorTroubleAgain.<init>(int)
at java.lang.Class.getConstructor0(Class.java:2706)
at java.lang.Class.getConstructor(Class.java:1657)
at ConstructorTroubleAgain.main(ConstructorTroubleAgain.java:26)
如果开发人员错误地认为反射会自动装箱或取消装箱,则可能会发生此异常。装箱(将 Primitives 转换为引用类型)仅在编译期间发生。没有机会反思此操作的发生,因此在定位构造函数时必须使用特定的类型。
$ java ConstructorTroubleAgain Object
Constructor passed Object
在这里,由于使用更具体的String
类型调用了newInstance()
,因此可以预期将调用带有String参数的构造函数。但是为时已晚!找到的构造函数已经是带有Object参数的构造函数。 newInstance()
不try进行方法解析;它仅对现有的构造函数对象进行操作。
Tip:
new
和Constructor.newInstance()之间的重要区别在于new
执行方法参数类型检查,装箱和方法解析。这些都没有反映出必须做出明确的选择。
try调用无法访问的构造方法时出现 IllegalAccessException
如果try调用私有或其他无法访问的构造函数,则可能引发IllegalAccessException。 ConstructorTroubleAccess示例说明了生成的堆栈跟踪。
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
class Deny {
private Deny() {
System.out.format("Deny constructor%n");
}
}
public class ConstructorTroubleAccess {
public static void main(String... args) {
try {
Constructor c = Deny.class.getDeclaredConstructor();
// c.setAccessible(true); // solution
c.newInstance();
// production code should handle these exceptions more gracefully
} catch (InvocationTargetException x) {
x.printStackTrace();
} catch (NoSuchMethodException x) {
x.printStackTrace();
} catch (InstantiationException x) {
x.printStackTrace();
} catch (IllegalAccessException x) {
x.printStackTrace();
}
}
}
$ java ConstructorTroubleAccess
java.lang.IllegalAccessException: Class ConstructorTroubleAccess can not access
a member of class Deny with modifiers "private"
at sun.reflect.Reflection.ensureMemberAccess(Reflection.java:65)
at java.lang.reflect.Constructor.newInstance(Constructor.java:505)
at ConstructorTroubleAccess.main(ConstructorTroubleAccess.java:15)
Tip:
存在访问限制,该访问限制可防止通常无法通过直接调用访问的构造函数的反射调用。 (这包括但不限于,一个单独的类中的私有构造函数和一个单独的私有类中的公共构造函数.)但是,声明Constructor
来扩展AccessibleObject,从而可以通过AccessibleObject.setAccessible()取消此检查。