02 捕获所有异常

1 捕获所有异常

通过 Exception 捕获所有异常，并调用它从其基类 Throwable 继承的方法进一步了解当然异常的类型。

package com.hcong.exceptions;

/**
 * @Classname ExceptionMethods
 * @Date 2023/4/15 16:45
 * @Created by HCong
 */
public class ExceptionMethods {
    public static void main(String[] args) {
        try {
            throw new NullPointerException("My Exception");
        } catch (Exception e) {
            System.out.println("Caught Exception");
            System.out.println(
                    "getMessage():" + e.getMessage());
            System.out.println(
                    "getLocalizedMessage():" + e.getLocalizedMessage());
            System.out.println("toString():" + e);
            System.out.print("printStackTrace():");
            e.printStackTrace(System.out);
        }
    }
}

Caught Exception
getMessage():My Exception
getLocalizedMessage():My Exception
toString():java.lang.NullPointerException: My Exception
printStackTrace():java.lang.NullPointerException: My Exception
	at com.hcong.exceptions.ExceptionMethods.main(ExceptionMethods.java:11)

2 栈轨迹

printStackTrace() 方法所提供的信息可以通过 getStackTrace() 方法来直接访问，这个方法将返回一个由栈轨迹中的元素所构成的数组，其中每一个元素都表示栈中的一桢。

package com.hcong.exceptions;

/**
 * @Classname WhoCalled
 * @Date 2023/4/15 16:50
 * @Created by HCong
 */
public class WhoCalled {
    static void f() {
        try {
            throw new Exception();
        } catch (Exception e) {
            for (StackTraceElement ste : e.getStackTrace()) {
                System.out.println(ste.getMethodName());
            }
        }
    }

    static void g() {
        f();
    }

    static void h() {
        g();
    }

    public static void main(String[] args) {
        f();
        System.out.println("=================");
        g();
        System.out.println("=================");
        h();
    }
}

f
main
=================
f
g
main
=================
f
g
h
main

元素 0 是栈顶元素，并且是调用序列中的最后一个方法调用（这个 Throwable 被创建和抛出之处）。数组中的最后一个元素和栈底是调用序列中的第一个方法调用。

3 重新抛出异常

将捕获的异常重新抛出，重抛异常会把异常抛给上一级环境中的异常处理程序，同一个 try 块的后续 catch 子句将被忽略。此外，值得注意的是：

• 如果只是把当前异常对象重新抛出，那么 printStackTrace()
  方法显示的将是原来异常抛出点的调用栈信息，而并非重新抛出点的信息
• 要想更新这个信息，可以调用 fillInStackTrace() 方法，这将返回一个Throwable 对象，它是通过把当前调用栈信息填入原来那个异常对象而建立的。

package com.hcong.exceptions;

/**
 * @Classname Rethrowing
 * @Date 2023/4/15 17:05
 * @Created by HCong
 */
public class Rethrowing {
    public static void f() throws Exception {
        System.out.println(
                "originating the exception in f()");
        throw new Exception("thrown from f()");
    }

    public static void g() throws Exception {
        try {
            f();
        } catch (Exception e) {
            System.out.println(
                    "Inside g(), e.printStackTrace()");
            e.printStackTrace(System.out);
            throw e;
        }
    }

    public static void h() throws Exception {
        try {
            f();
        } catch (Exception e) {
            System.out.println(
                    "Inside h(), e.printStackTrace()");
            e.printStackTrace(System.out);
            throw (Exception) e.fillInStackTrace();
        }
    }

    public static void main(String[] args) {
        try {
            g();
        } catch (Exception e) {
            System.out.println("main: printStackTrace()");
            e.printStackTrace(System.out);  // 栈轨迹与 g() 中的一致
        }
        System.out.println("=============================================");
        try {
            h();
        } catch (Exception e) {
            System.out.println("main: printStackTrace()");
            e.printStackTrace(System.out);  // 栈轨迹与 h() 中的不一致
        }
    }
}

originating the exception in f()
Inside g(), e.printStackTrace()
java.lang.Exception: thrown from f()
	at com.hcong.exceptions.Rethrowing.f(Rethrowing.java:12)
	at com.hcong.exceptions.Rethrowing.g(Rethrowing.java:17)
	at com.hcong.exceptions.Rethrowing.main(Rethrowing.java:39)
main: printStackTrace()
java.lang.Exception: thrown from f()
	at com.hcong.exceptions.Rethrowing.f(Rethrowing.java:12)
	at com.hcong.exceptions.Rethrowing.g(Rethrowing.java:17)
	at com.hcong.exceptions.Rethrowing.main(Rethrowing.java:39)
=============================================
originating the exception in f()
Inside h(), e.printStackTrace()
java.lang.Exception: thrown from f()
	at com.hcong.exceptions.Rethrowing.f(Rethrowing.java:12)
	at com.hcong.exceptions.Rethrowing.h(Rethrowing.java:28)
	at com.hcong.exceptions.Rethrowing.main(Rethrowing.java:46)
main: printStackTrace()
java.lang.Exception: thrown from f()
	at com.hcong.exceptions.Rethrowing.h(Rethrowing.java:33)
	at com.hcong.exceptions.Rethrowing.main(Rethrowing.java:46)

Process finished with exit code 0