异常处理

异常分类

异常对象都是派生于Throwable类的一个实例

异常结构

• Throwable

  • Error
  • Exception
    • IOException
    • RuntimeException

• 派生于Error和RuntimeException的称为非受查异常，其他的称为受查异常

// 顶层异常类
public class Throwable implements Serializable {}

// Java运行时系统内部错误和资源耗尽
public class Error extends Throwable {}

// 异常
public class Exception extends Throwable {}

// 运行时异常
public class RuntimeException extends Exception {}

// IO异常
public class IOException extends Exception {}

• 常见Exception

EOFException
FileNotFoundException
IOException
ClassNotFoundException
NoSuchFieldException
NoSuchMethodException
ReflectiveOperationException
RuntimeException

• 常见RuntimeException

ArithmeticException.java
ArrayIndexOutOfBoundsException.java
ArrayStoreException.java
ClassCastException.java
EnumConstantNotPresentException.java
IllegalArgumentException.java
IllegalMonitorStateException.java
IllegalStateException.java
IllegalThreadStateException.java
IndexOutOfBoundsException.java
NegativeArraySizeException.java
NullPointerException.java
NumberFormatException.java
SecurityException.java
StringIndexOutOfBoundsException.java
TypeNotPresentException.java
UnsupportedOperationException.java

• 子类抛出的异常不能比父类更通用
• 声明异常使用throws

public FileInputStream(File file) throws FileNotFoundException {}

• 抛出异常使用throw

 throw new NullPointerException();

创建异常类

• 应该包含无参构造和带有详细信息的参数构造器

public class ExceptionDemo1 extends Exception {

    public ExceptionDemo1() {
    }

    // 能够打印详细信息
    public ExceptionDemo1(String message) {
        super(message);
    }
}

捕获异常

try {
    // error
} catch() {
    // process error
}

抛出异常和异常链

• 抛出原始异常，可以使用initCause设置
• 获取原始异常getCause

public static void main(String[] args) {
    try {
        b();
    } catch (Throwable e) {
        System.out.println("error = " + e.getMessage());
        System.out.println("init cause = " + e.getCause());
    }
}

public static void a() {
    throw new NullPointerException();
}

public static void b() throws Throwable {
    try {
        a();
    } catch (Exception e) {
        Throwable th = new Exception("b methods error");
        // 将e设置为原始异常
        th.initCause(e);
        // 等同于
        // Throwable th = new Exception("b methods error", e);
        throw th;
    }
}

finally

• 最终一定会被执行的代码块

try {
    
} catch() {
    
} finally {
    
}

• finally出现return时，try块的返回无效，因为finally是一定会被执行的，在try中return前就已经执行，所以先返回

public static void main(String[] args) {
    System.out.println(a()); // 2
}

static int a() {
    try {
        return 1;
    } finally {
        return 2;
    }
}

try-with-resource

• try块退出时，会自动调用close方法

public static void main(String[] args) {
    try (FileInputStream in = new FileInputStream("");
         FileOutputStream out = new FileOutputStream("")) {

    } catch (Exception e) {

    }
}

堆栈轨迹元素

• 打印异常堆栈printStackTrace
• 获取堆栈数组信息getStackTrace

public static void main(String[] args) {
    try {
        a();
    } catch (Exception e) {
        e.printStackTrace();
        System.out.println("--------------");
        StackTraceElement[] stackTrace = e.getStackTrace();
        Stream.of(stackTrace).forEach(System.out::println);
    }
}

static void a() {
    throw new NullPointerException();
}

java.lang.NullPointerException
	at com.java.practice.exception.ExceptionDemo4.a(ExceptionDemo4.java:26)
	at com.java.practice.exception.ExceptionDemo4.main(ExceptionDemo4.java:16)
--------------
com.java.practice.exception.ExceptionDemo4.a(ExceptionDemo4.java:26)
com.java.practice.exception.ExceptionDemo4.main(ExceptionDemo4.java:16)