Java异常原因及处理

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Java常见异常

classDiagram
  Object <|-- Throwable
  Throwable <|-- Error
  Throwable <|-- Exception
  Error <|-- OutOfMemoryError
  Error <|-- NoClassDefFoundError
  Error <|-- StackOverflowError
  Exception <|-- IOException
  Exception <|-- RuntimeException
  RuntimeException <|-- NullPointerException
  RuntimeException <|-- IndexOutOfBoundsException

NoClassDefFoundError 异常原因处理

类初始化失败

如果一个类初始化失败，在第一次失败的时候会报错java.lang.ExceptionInInitializerError，第二次就会java.lang.NoClassDefFoundError

今天碰到这个问题了，查了一下午（2023-05-05），fuck

package com.example.demo;

public class Main {
    public static void main(String[] args) {
        try {
            new ClassNotDefErrorFromClassLoad();
        } catch (Throwable e) {
            e.printStackTrace();
        }
        new ClassNotDefErrorFromClassLoad();
    }
}

class ClassNotDefErrorFromClassLoad {
    private static final String ERROR = String.valueOf(1 / 0);
}

java.lang.ExceptionInInitializerError
    at com.example.demo.Main.main(Main.java:6)
Caused by: java.lang.ArithmeticException: / by zero
    at com.example.demo.ClassNotDefErrorFromClassLoad.<clinit>(Main.java:15)
    ... 1 more
Exception in thread "main" java.lang.NoClassDefFoundError: Could not initialize class com.example.demo.ClassNotDefErrorFromClassLoad
    at com.example.demo.Main.main(Main.java:10)

class文件丢失

对下面这个类，启动后程序运行正常

package com.example.demo;

public class Main {
    public static void main(String[] args) {
        new ClassNotDefErrorFromClassNotFound();
    }
}

class ClassNotDefErrorFromClassNotFound {
}

Process finished with exit code 0

我们观察编译的结果，发现两个class文件

$ tree target/
target/
|-- classes
|   |-- application.properties
|   `-- com
|       `-- example
|           `-- demo
|               |-- ClassNotDefErrorFromClassNotFound.class
|               `-- Main.class
|-- generated-sources
|   `-- annotations
|-- generated-test-sources
|   `-- test-annotations
`-- test-classes

这时候我们删除ClassNotDefErrorFromClassNotFound.class，然后运行代码，发现报错NoClassDefFoundError

Exception in thread "main" java.lang.NoClassDefFoundError: com/example/demo/ClassNotDefErrorFromClassNotFound
    at com.example.demo.Main.main(Main.java:5)
Caused by: java.lang.ClassNotFoundException: com.example.demo.ClassNotDefErrorFromClassNotFound
    at java.net.URLClassLoader.findClass(URLClassLoader.java:382)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    ... 1 more

Process finished with exit code 1

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