Day3-JAVA—异常

异常是什么？

• 程序运行时有着太多的不确定性，比如写好一段读取文件的程序，下一次运行时文件被人给删除了。程序要第一时间给程序员这个文件不存在的消息。这个因为不确定性而导致程序无法运行时给出的消息，就是我们所说的异常。
• Java中万事万物皆对象，因此这个异常消息，也被Java定义成了对象(Exception)

异常体系图

异常分类

• 受检异常。。。。啥含义？
• 运行时异常。。。啥含义？

异常常见方法

• getMessage()
  • 返回抛出异常的详细信息
• printStackTrace()
  • 打印异常信息到标准输出流上

常见受检异常

常见运行时异常

try-catch-finally

• 代码块

Demo1.java

package course.basic.exception;

import java.io.IOException;

public class Demo1 {

    public static void main(String[] args) throws IOException {
        testRuntimeException();

foo();
}

    public static void foo() throws IOException {
        testCheckedException();
}

    public static void testRuntimeException() throws NullPointerException {
        System.out.println("Demo1.testRuntimeException");
}

    public static void testCheckedException() throws IOException {
        System.out.println("Demo1.testCheckedException");
}
}

Demo2.java

import java.io.*;

public class Demo2 {

    public static void main(String[] args) {
        // foo1();

        // foo2();

/**
         * jdk 1.7之前
* try{
         *
         * }catch(XxxException e){
         *
         * }finally{
         *
         * }
         *
         * jdk1.7新特性
* try(){
         *
         * }catch(XxxException e){
         *
         * }
         *
         *
         */
}

    public static void foo3() {
        // try() 写法必须是类要实现自 Closeable 接口
// try (Foo2 foo = new Foo2()) {
        //
        // } catch (Exception e) {
        //     e.printStackTrace();
        // }
}

    static class Foo1 implements Closeable {

        @Override
public void close() throws IOException {

        }
    }

    static class Foo2 {

    }

    public static void foo2() {
        String path = "/Users/haoc/course/code/cakes-course/basic/src/main/java/course/basic/exception/Demo2.jaa";

        try (BufferedReader reader = new BufferedReader(new FileReader(new File(path)))) {
            String line = null;
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
}
        } catch (FileNotFoundException e) {
            System.out.println("getMessage:" + e.getMessage());
System.out.println("---------");
e.printStackTrace();
//  System.out.println("Demo2.FileNotFoundException");
} catch (IOException e) {
            e.printStackTrace();
System.out.println("Demo2.IOException");
}
    }

    public static void foo1() {
        String path = "/Users/haoc/course/code/cakes-course/basic/src/main/java/course/basic/exception/Demo2.jaa";

BufferedReader reader = null;
        try {
            reader = new BufferedReader(new FileReader(new File(path)));

String line = null;
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
}
        } catch (FileNotFoundException e) {
            System.out.println("getMessage:" + e.getMessage());
System.out.println("---------");
e.printStackTrace();
//  System.out.println("Demo2.FileNotFoundException");
} catch (IOException e) {
            e.printStackTrace();
System.out.println("Demo2.IOException");
} finally {
            if (null != reader) {
                try {
                    reader.close();
} catch (IOException e) {
                    e.printStackTrace();
}
            }
        }
    }

}

Demo3.java

import java.io.Closeable;
import java.io.IOException;

/**
 *
 */
public class Demo3 {
    public static void main(String[] args) {
        try (Foo1 foo = new Foo1()) {

        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    static class Foo1 implements Closeable {

        @Override
        public void close() throws IOException {

        }
    }
}

Demo4.java

public class Demo4 {
    public static void main(String[] args) {

    }

    public static void foo1() {
        // send http request

        try {
            // timeout 异常  1
            // String response = HttpClient.doPost(url,params);
            // response handle
            // response.data.bizCode = 余额不足 异常  2
            // response.data.bizCode = 支付系统故障   3
            // } catch (TimeoutEx e) {
            //     // retry
            // } catch (BalanceEx e) {
            //     // ignore
            // } catch (SysEx e) {
            //     // send alarm
            //     throw e;
            // } }
        } catch (Exception e) {
            // log.error("failed. push data. 余额不足") // 三类异常

            StackTraceElement[] stackTraceArr = e.getStackTrace();
            for (StackTraceElement stackTraceElement : stackTraceArr) {
                stackTraceElement.getMethodName();
                stackTraceElement.getClassName();
                stackTraceElement.getLineNumber();
            }

            handleException(e);
        }
    }

    static void handleException(Exception e) {
        if (e instanceof TimeoutEx) {
            // retry
        }

        if (e instanceof BalanceEx) {
            // ignore
        }

        if (e instanceof SysEx) {
            // log.error()
        }
    }

    /**
     * 自定义异常
     */
    static class TimeoutEx extends RuntimeException {
        public TimeoutEx() {
        }

        public TimeoutEx(String message) {
            super(message);
        }

        public TimeoutEx(String message, Throwable cause) {
            super(message, cause);
        }

        public TimeoutEx(Throwable cause) {
            super(cause);
        }

        public TimeoutEx(String message, Throwable cause, boolean enableSuppression, boolean writableStackTrace) {
            super(message, cause, enableSuppression, writableStackTrace);
        }
    }

    static class BalanceEx extends RuntimeException {

    }

    static class SysEx extends RuntimeException {

    }
}