异常处理 Java

What is an Exception in Java?

An 异常在 Java is an event that interrupts the execution of program instructions and disturbs the normal flow of program execution. It is an object that wraps an error event information that occurred within a method, and it is passed to the runtime system. In Java, exceptions are mainly used to signal different types of error conditions so that developers can respond to them in a controlled way.

There are two broad categories of errors in Java:

1. 编译时错误
2. 运行时错误

Compile time errors can be classified into two further types:

• 语法错误
• 语义错误

语法错误示例：

而不是声明 int a; 你错误地将其声明为 in a;, for which the compiler will throw an error.

Semantic Errors Example: You have declared a variable int a; 几行代码之后你再次声明一个整数为 int a;。编译代码时，所有这些错误都会突出显示。

运行时错误示例

A Runtime error is referred to as an 特殊课程. It is any event that interrupts the normal flow of program execution. Examples of exceptions include arithmetic exception, NullPointerException, divide by zero exception, and many more. Exceptions in Java are conditions that are often outside a developer’s direct control.

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Why do we need Exception Handling?

Suppose you have coded a program to access a server. Things work fine while you are developing the code in a local environment with controlled conditions.

During an actual production run, the server may be down. When your program tries to access it, an exception is raised, and without proper handling the program will terminate abruptly.

How to Handle an Exception in Java

So far we have seen that an exception is often beyond a developer’s control. However, blaming code failure on environmental issues is not a real solution. You need robust programming that takes care of exceptional situations. Such code is known as an Exception Handler.

In our example, good exception handling would mean that when the primary server is down, the program automatically connects to a backup server.

To implement this, you could write code to connect to the server using traditional if and else conditions. You will check if the server is down, and if yes, you will write code to connect to the backup server. Such organization of code using “if” and “else” branches is not effective when your code has multiple Java 例外 处理。

class connect{
	if(Server Up){
	 // code to connect to server
	}
	else{
	 // code to connect to BACKUP server
	}
}

Try Catch Block in Java

Java provides an inbuilt exception handling mechanism that separates normal logic from error recovery logic.

1. 正常代码进入 TRY 块。
2. 异常处理代码进入 CATCH 块。

In our example, the TRY block will contain the code to connect to the server. The CATCH block will contain the code to connect to the backup server. In case the server is up, the code in the CATCH block will be ignored. In case the server is down, an exception is raised, and the code in the catch block will be executed.

So, this is how an exception is handled in Java.

Syntax for using try and catch

try{
    statement(s)
}
catch (exceptiontype name){
	statement(s)
}

Example of Try Catch Block

步骤1） 将以下代码复制到编辑器中。

class JavaException {
   public static void main(String args[]){
      int d = 0;
      int n = 20;
      int fraction = n/d;
     System.out.println("End Of Main");
   }
}

步骤2） Save the file and compile the code. Run the program using the command, java Java例外。

步骤3） An Arithmetic Exception, divide by zero, is shown for line 5, and line 6 is never executed.

步骤4） Now let us examine how try and catch will help us to handle this exception. We will put the exception causing line of code into a 尝试 块，然后是 捕捉 块。将以下代码复制到编辑器中。

class JavaException {
 public static void main(String args[]) {
  int d = 0;
  int n = 20;
  try {
   int fraction = n / d;
   System.out.println("This line will not be Executed");
  } catch (ArithmeticException e) {
   System.out.println("In the catch Block due to Exception = " + e);
  }
  System.out.println("End Of Main");
 }
}

步骤5） Save, compile, and run the code. You will get the following output.

As you observe, the exception is handled, and the last line of code is also executed. Also, note that line 7 will not be executed because 一旦引发异常，控制流就会跳转到 catch 块。

注意： The ArithmeticException object “e” carries information about the exception that has occurred, which can be useful when taking recovery actions.

Java Exception Class Hierarchy

After one catch statement executes, the others are bypassed, and execution continues after the try and catch block. Nested catch blocks must follow the exception hierarchy from the most specific subclass to the most general parent class.

• 所有异常类 Java extend the class Throwable. Throwable has two subclasses, Error and Exception.
• The Error class defines the exceptions or problems that are not expected to occur under normal circumstances in your program, for example, memory error, hardware error, or JVM error.
• The Exception class represents the exceptions that can be handled by your program, and your program can recover from these exceptions using try and catch blocks.
• A RuntimeException is a subclass of the Exception class. Exceptions of this type represent unchecked exceptions that occur at runtime and cannot be detected at compile time. Good examples include divide by zero exception and null pointer exception.
• IOException is generated during input and output operations.
• InterruptedException in Java is generated during multithreaded operations when a thread is interrupted.

Example: Nesting of try and catch blocks

步骤1） 将以下代码复制到编辑器中。

class JavaException {
 public static void main(String args[]) {
  try {
   int d = 1;
   int n = 20;
   int fraction = n / d;
   int g[] = {
    1
   };
   g[20] = 100;
  }
  /*catch(Exception e){
  	System.out.println("In the catch block due to Exception = "+e);
  }*/
  catch (ArithmeticException e) {
   System.out.println("In the catch block due to Exception = " + e);
  } catch (ArrayIndexOutOfBoundsException e) {
   System.out.println("In the catch block due to Exception = " + e);
  }
  System.out.println("End Of Main");
 }
}

步骤2） Save the file and compile the code. Run the program using the command, java的 Java特殊课程.

步骤3） An ArrayIndexOutOfBoundsException is generated. Change the value of int d to 0. Save, compile, and run the code.

步骤4） An ArithmeticException must be generated.

步骤5） Uncomment line 10 to line 12. Save, compile, and run the code.

Step 6) Compilation Error? This is because Exception is the base class of ArithmeticException. Any exception that is raised by ArithmeticException can also be handled by the Exception class. So the catch block of ArithmeticException will never get a chance to execute, which makes it redundant. Hence the compilation error.

Java 最终块

finally 块是 无论是否引发异常都执行 在 try 块中。它是 可选 to use with a try block. The finally block is the right place to release resources such as database connections, files, or network sockets.

 try {
  statement(s)
 } catch (ExceptiontType name) {

  statement(s)

 } finally {

  statement(s)

 }

If an exception is raised in the try block, the finally block is executed after the catch block completes.

Example of finally Block

步骤1） 将以下代码复制到编辑器中。

class JavaException {
   public static void main(String args[]){
    try{
       int d = 0;
       int n =20;
       int fraction = n/d;
    }
  catch(ArithmeticException e){
    System.out.println("In the catch block due to Exception = "+e);
  }
  finally{
	System.out.println("Inside the finally block");
  }
}
}

步骤2） Save, compile, and run the code.

步骤3） Expected output: The finally block is executed even though an exception is raised.

步骤4） Change the value of variable d = 1. Save, compile, and run the code, and observe the output.

最佳实践 Java 异常处理

Following a few proven rules will help you keep your Java code resilient, readable, and easy to debug:

• Catch the most specific exception first, then the more general ones, to preserve the integrity of the exception hierarchy.
• Avoid empty catch blocks. At a minimum, log the exception so the root cause is never lost.
• Use the finally block or try with resources to release files, sockets, and database connections.
• Throw checked exceptions only when the caller can reasonably recover, and use unchecked exceptions for programming errors.
• Wrap low level exceptions in a meaningful business exception to keep API contracts clean.
• Never swallow InterruptedException silently in multithreaded code, restore the interrupt flag instead.

常见问题