吃透面试二：class.newInstance()方法为什么从java9开始弃用

我们先看一下官方说明

     * @deprecated This method propagates any exception thrown by the
     * nullary constructor, including a checked exception.  Use of
     * this method effectively bypasses the compile-time exception
     * checking that would otherwise be performed by the compiler.
     * The {
     @link
     * java.lang.reflect.Constructor#newInstance(java.lang.Object...)
     * Constructor.newInstance} method avoids this problem by wrapping
     * any exception thrown by the constructor in a (checked) {
     @link
     * java.lang.reflect.InvocationTargetException}.

大致意思是：

class.newInstance()方法传播由空构造函数引发的任何异常，包括已检查的异常。
这种方法的使用有效地绕过了编译时异常检查，否则该检查将由编译器执行。

clazz.newInstance()方法由clazz.getDeclaredConstructor().newInstance()方法代替，该方法通过将构造函数抛出的任何异常包装在（InvocationTargetException）中来避免此问题。

也就是说使用class.newInstance()方法时由默认构造函数中抛出的异常，此方法检查不到，如下例子：

public class ClassInstanceTest {
     

    public ClassInstanceTest() throws IOException {
     
        System.out.println("无参构造");
        throw new IOException();
    }

    public static void main(String[] args) {
     
        try {
     
            ClassInstanceTest classInstanceTest = ClassInstanceTest.class.newInstance();
        } catch (InstantiationException e) {
     
            System.out.println("InstantiationException");
            e.printStackTrace();
        } catch (IllegalAccessException e) {
     
            System.out.println("IllegalAccessException");
            e.printStackTrace();
        }
    }
}

运行输出：

无参构造
Exception in thread "main" java.io.IOException
	at ClassInstanceTest.<init>(ClassInstanceTest.java:9)
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
	at java.base/java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:500)
	at java.base/java.lang.reflect.ReflectAccess.newInstance(ReflectAccess.java:166)
	at java.base/jdk.internal.reflect.ReflectionFactory.newInstance(ReflectionFactory.java:404)
	at java.base/java.lang.Class.newInstance(Class.java:590)
	at ClassInstanceTest.main(ClassInstanceTest.java:14)

我们看到，没有被异常检查捕获到吧。

接下来我们使用clazz.getDeclaredConstructor().newInstance()方法：

public class ClassInstanceTest {
     

    public ClassInstanceTest() throws IOException {
     
        System.out.println("无参构造");
        throw new IOException();
    }

    public static void main(String[] args) {
     
        try {
     
            Class clazz = ClassInstanceTest.class;
            clazz.getDeclaredConstructor().newInstance();
        } catch (InstantiationException e) {
     
            System.out.println("InstantiationException");
            e.printStackTrace();
        } catch (IllegalAccessException e) {
     
            System.out.println("IllegalAccessException");
            e.printStackTrace();
        } catch (InvocationTargetException e) {
     
            System.out.println("InvocationTargetException");
            e.printStackTrace();
        } catch (NoSuchMethodException e) {
     
            System.out.println("NoSuchMethodException");
            e.printStackTrace();
        }
    }
}

运行输出：

无参构造
InvocationTargetException
java.lang.reflect.InvocationTargetException
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
	at java.base/java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:500)
	at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:481)
	at ClassInstanceTest.main(ClassInstanceTest.java:15)
Caused by: java.io.IOException
	at ClassInstanceTest.<init>(ClassInstanceTest.java:9)
	... 6 more

可以看到被InvocationTargetException异常捕获到了。