Java collection processing / null value processing / exception processing, use experience sharing!

Java technology stack 2021-02-23 11:27:42
java collection processing null value


1. Arrays.asList

Business development , We often convert the original array to List Class data structure , To carry on with all kinds of Stream operation

  • Arrays.asList Cannot convert array of base type , have access to Arrays.stream To switch
  • Arrays.asList Back to list Adding and deleting operations are not supported , It returned List yes Arrays The inner class of ArrayList. Internally inherited from AbstractList, There is no overriding parent add Method
  • The modification of the original array will affect the one we get List
    • ArrayList It's actually using the original array , So when you use it , It's best to reuse New ArrayList To decouple

2. Null processing

2.1 NullPointerException

  • Possible scenarios

    • The parameter value is Integer And so on , When using, there is an exception of null pointer due to automatic unpacking
    • String comparison
    • ConcurrentHashMap This container does not support Key and Value by null, Force put null Of key or Value A null pointer exception will occur
    • Method or remote service list yes null, Call directly without null , A null pointer exception occurred
    • Method of joint level call null check
  • best practice

    • string.equalsTo(variableName)
    • Optional.ofNullable()
    • orElse()

3. exception handling

3.1 Consider exception handling at the business code level

  • Most business applications adopt a three-tier architecture
    • Controller layer
      • Responsible for information collection , Parameter checking , Data adaptation front end of transformation service layer processing , Light business logic
      • Controller Capture exception , Then you need to give user-friendly tips
    • Service layer
      • Responsible for the core business logic , Including external service calls , Access database , Cache handling , Message processing, etc
      • It usually involves database transactions , Exception not suitable for capture , Otherwise, the transaction cannot be rolled back automatically
    • Repository layer
      • Responsible for data access implementation , Generally, there is no business logic
      • Do it according to the situation , Downgrade , Or turn it into a friendly exception
  • Exception handling at the framework level
    • Try not to do abnormal automation at the framework level , Unified treatment
    • The framework should do all the work , If the exception goes up to the top level and the logic still can't handle it , Exception conversion can be done in a unified way
      • @RestControllerAdvice
      • @ExceptionHandler

3.2 Don't eat it raw

After catching the exception, it should not be eaten raw , Because if the swallowing exception is not handled properly , appear Bug It's going to be hard to find .

There needs to be an appropriate transformation into a user-friendly exception , Or at least warn, error It's up to you log

3.3 Keep the original information

After catching the exception , Be sure to remember in log Or record the original exception information in the thrown out exception

catch (IOException e) {
// Only exception messages are kept , There are no records on the stack 
log.error(" File read error , {}", e.getMessage());
throw new RuntimeException(" Please try again later ");
}
catch (IOException e) {
throw new RuntimeException(" Please try again later ", e);
}123456789

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3.4 look out finally The abnormal + try with resources

Note that exceptions may also occur during resource release processing and other finishing operations , This time , If try block Logic and finnally If logic has exception throw ,try The exception will be finnally The exception in the , It makes the problem very unobtrusive .

@GetMapping("wrong")
public void wrong() {
try {
log.info("try");
// Abnormal loss 
throw new RuntimeException("try");
} finally {
log.info("finally");
throw new RuntimeException("finally");
}
}12345678910

For implemented AutoCloseable Interface resources , have access to try-with-resources To release resources , Is in the try Statement with resources in

try catch finally vs try with resources

Scanner scanner = null;
try {
scanner = new Scanner(new File("test.txt"));
while (scanner.hasNext()) {
System.out.println(scanner.nextLine());
}
} catch (FileNotFoundException e) {
e.printStackTrace();
} finally {
if (scanner != null) {
scanner.close();
}
}
try (Scanner scanner = new Scanner(new File("test.txt"))) {
while (scanner.hasNext()) {
System.out.println(scanner.nextLine());
}
} catch (FileNotFoundException fnfe) {
fnfe.printStackTrace();
}
123456789101112131415161718192021

3.5 Exception handling for thread pool tasks

Set a custom exception handler as a minimum , For example, when declaring a thread pool, customize the uncapped exception handler of the thread pool :

new ThreadFactoryBuilder()
.setNameFormat(prefix+"%d")
.setUncaughtExceptionHandler((thread, throwable)-> log.error("ThreadPool {} got exception", thread, throwable))
.get()1234

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Reference

  1. https://www.baeldung.com/java-try-with-resources
  2. https://time.geekbang.org/column/article/220230

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