Design and application of immutable classes

1、 The problem of date conversion

@Slf4j
public class Test {
    

    @SneakyThrows
    public static void main(String[] args) {
    
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        for (int i = 0; i < 10; i++) {
    
            new Thread(() -> {
    
                try {
    
                    log.info("{}", sdf.parse("1951-04-21"));
                } catch (Exception e) {
    
                    log.info("{}", e);
                }
            }).start();
        }
    }
}

The above code is prone to the following errors , as a result of SimpleDateFormat Not thread safe , Is a mutable class , That is, its internal state is variable

I want to solve this problem , Of course, the conventional practice is to lock , But locking will inevitably lead to performance degradation

2、 The use of immutable classes

If you can guarantee the internal state of an object （ attribute ） Cannot be modified , So it's thread safe , Because there is no concurrent modification ！ In fact, such objects are Java There are a lot of , For example, in Java 8 after , Provides a new date formatting class DateTimeFormatter , It is an immutable class , So it is a thread safe class

DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy-MM-dd");
for (int i = 0; i < 10; i++) {
    
    new Thread(() -> {
    
        LocalDate date = dtf.parse("2022-02-11", LocalDate::from);
        log.info("{}", date);
    }).start();
}

3、 The design of immutable classes

We use String Class, for example , Explain the elements of immutable design , Observing the source code is not difficult to find , This kind 、 All properties in the class are final Of ：

• Properties with final Modification ensures that the property is read-only , Do not modify
• Class with final Modification ensures that methods in this class cannot be overridden , Prevent subclasses from inadvertently destroying immutability

and , All the modification methods , Such as substring etc. , Its interior is also newly created String , And copy the original character array , This means of avoiding sharing by creating replica objects is called 【 Protective copy （defensive copy）】