Problems caused by using global variables
There is a classic interview question: the address of the global variable List modified by static final is unchanged, but the content is variable.
Preface
I encountered this problem at work recently: after each reading, processing will be performed. Eventually the List becomes larger and larger. And after restarting, the data will be reset.
1: Problem code
@RestController
@RequestMapping(value = "demo")
public class DemoController {
private static final List<String> STR_LIST = Lists.newArrayList("1", "2", "3", "4");
@PostMapping(value = "m1", name = "m1")
public void methodOne(@RequestBody JSONObject jsonObject) {
List<String> strList = STR_LIST;
strList.add(String.valueOf(strList.size() + 1));
System.out.println(STR_LIST);
}
}
Results of the:
[1, 2, 3, 4, 5]
[1, 2, 3, 4, 5, 6]
[1, 2, 3, 4, 5, 6, 7]
[1, 2, 3, 4, 5, 6, 7, 8]
[1, 2, 3, 4, 5, 6, 7, 8, 9]
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
Two: Correct the code
@RestController
@RequestMapping(value = "demo")
public class DemoController {
private static final List<String> STR_LIST = Lists.newArrayList("1", "2", "3", "4");
@PostMapping(value = "m1", name = "m1")
public void methodOne(@RequestBody JSONObject jsonObject) {
List<String> strList = Lists.newArrayList(STR_LIST);
strList.add(String.valueOf(strList.size() + 1));
System.out.println(STR_LIST);
}
}
Results of the:
[1, 2, 3, 4]
[1, 2, 3, 4]
[1, 2, 3, 4]
[1, 2, 3, 4]
[1, 2, 3, 4]
Summarize
When using global variables, it is recommended to use new to receive them instead of directly equaling them.
List<String> strList = Lists.newArrayList(STR_LIST);