零散的Java知识记录 3
LiuYang 2021/11/19 Java
# List操作
# Arrays.asList
- 不能直接使用 Arrays.asList 来转换基本类型数组。
- Arrays.asList 返回的 List 不支持增删操作。
- 对原始数组的修改会影响到我们获得的那个 List。
# List.subList
- 使用subList操作的数据源无法得到回收，因为它始终被 subList 方法返回的 List 强引用
- 对subList得到的List进行add等操作，回与源数据之间进行相互影响。
# Null
# POJO 中属性的 null
- 对于 JSON 到 DTO 的反序列化过程，null 的表达是有歧义的，客户端不传某个属性，或者传 null，这个属性在 DTO 中都是 null。
- POJO 中的字段有默认值。如果客户端不传值，就会赋值为默认值，导致创建时间也被 更新到了数据库中。
- 注意字符串格式化时可能会把 null 值格式化为 null 字符串。
# MySQL 中有关 NULL
- MySQL 中 sum 函数没统计到任何记录时，会返回 null 而不是 0,可以使用 IFNULL 函数把 null 转换为 0;
- MySQL 中 count 字段不统计 null 值，COUNT(*) 才是统计所有记录数量的正确方 式。
- MySQL 中 =NULL 并不是判断条件而是赋值，对 NULL 进行判断只能使用 IS NULL 或 者 IS NOT NULL。
# Exception
- 统一的方式进行异常转换，比如通过 @RestControllerAdvice + @ExceptionHandler，来捕获“未处理”异常
- Repository 层出现异常或许可以忽略，或许可以降级，或许需要转化为一个友好的异常。如果一律捕获异常仅记录日志，很可能业务逻辑已经出错，而用户和程序本身完全感知不到。
- Service 层往往涉及数据库事务，出现异常同样不适合捕获，否则事务无法自动回滚。此外 Service 层涉及业务逻辑，有些业务逻辑执行中遇到业务异常，可能需要在异常后转入分支业务流程。如果业务异常都被框架捕获了，业务功能就会不正常。
- 如果下层异常上升到 Controller 层还是无法处理的话，Controller 层往往会给予用户友好提示，或是根据每一个 API 的异常表返回指定的异常类型，同样无法对所有异常一视同仁。
- 不要丢失异常原始信息，可以用类似throw new RuntimeException("系统忙请稍后再试", e);来进行转换。
- 捕获异常不要不做操作，哪怕输入日志
- 抛出异常需要执行信息。
- 不要定义静态的异常。
# 异常处理
除了通过日志正确记录异常原始信息外，通常还有 三种处理模式:
- 转换，即转换新的异常抛出。对于新抛出的异常，最好具有特定的分类和明确的异常消息，而不是随便抛一个无关或没有任何信息的异常，并最好通过 cause 关联老异常。
- 重试，即重试之前的操作。比如远程调用服务端过载超时的情况，盲目重试会让问题更严重，需要考虑当前情况是否适合重试。
- 恢复，即尝试进行降级处理，或使用默认值来替代原始数据。
# finally 中的异常
try中异常被finally中异常覆盖的情况
public void wrong() {
try {
log.info("try");
throw new RuntimeException("try");
} finally {
log.info("finally");
throw new RuntimeException("finally");
}
}
1
2
3
4
5
6
7
8
9
2
3
4
5
6
7
8
9
--------------------------------------------------------
2022-01-13 00:27:12.374 INFO 43202 --- [nio-8080-exec-1] c.l.bugs.l12.FinallyIssueController : try
2022-01-13 00:27:12.374 INFO 43202 --- [nio-8080-exec-1] c.l.bugs.l12.FinallyIssueController : finally
2022-01-13 00:27:12.375 ERROR 43202 --- [nio-8080-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet] : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.RuntimeException: finally] with root cause
java.lang.RuntimeException: finally
at com.liuyang19900520.bugs.l12.FinallyIssueController.wrong(FinallyIssueController.java:37) ~[main/:na]
... ...
1
2
3
4
5
6
7
8
2
3
4
5
6
7
8
解决方法1：finally 代码块自己负责异常捕获和处理
public void right() {
try {
log.info("try");
throw new RuntimeException("try");
} finally {
log.info("finally");
try {
throw new RuntimeException("finally");
} catch (Exception ex) {
log.error("finally", ex);
}
}
}
1
2
3
4
5
6
7
8
9
10
11
12
13
2
3
4
5
6
7
8
9
10
11
12
13
--------------------------------------------------------
2022-01-13 00:31:28.491 INFO 43202 --- [nio-8080-exec-1] c.l.bugs.l12.FinallyIssueController : try
2022-01-13 00:31:28.491 INFO 43202 --- [nio-8080-exec-1] c.l.bugs.l12.FinallyIssueController : finally
2022-01-13 00:31:28.491 ERROR 43202 --- [nio-8080-exec-1] c.l.bugs.l12.FinallyIssueController : finally
java.lang.RuntimeException: finally
at com.liuyang19900520.bugs.l12.FinallyIssueController.right(FinallyIssueController.java:49) ~[main/:na]
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
... ...
2022-01-13 00:31:28.492 ERROR 43202 --- [nio-8080-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet] : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.RuntimeException: try] with root cause
java.lang.RuntimeException: try
at com.liuyang19900520.bugs.l12.FinallyIssueController.right(FinallyIssueController.java:45) ~[main/:na]
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
... ...
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
解决方法2：使用 addSuppressed 方法把 finally 中的异常 附加到主异常上
public void right2() throws Exception {
Exception e = null;
try {
log.info("try");
throw new RuntimeException("try");
} catch (Exception ex) {
e = ex;
} finally {
log.info("finally");
try {
throw new RuntimeException("finally");
} catch (Exception ex) {
if (e != null) {
e.addSuppressed(ex);
} else {
e = ex;
}
}
}
throw e;
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
--------------------------------------------------------
2022-01-13 00:34:27.291 INFO 43202 --- [nio-8080-exec-1] c.l.bugs.l12.FinallyIssueController : try
2022-01-13 00:34:27.291 INFO 43202 --- [nio-8080-exec-1] c.l.bugs.l12.FinallyIssueController : finally
2022-01-13 00:34:27.292 ERROR 43202 --- [nio-8080-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet] : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.RuntimeException: try] with root cause
java.lang.RuntimeException: try
at com.liuyang19900520.bugs.l12.FinallyIssueController.right2(FinallyIssueController.java:61) ~[main/:na]
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
... ...
Suppressed: java.lang.RuntimeException: finally
at com.liuyang19900520.bugs.l12.FinallyIssueController.right2(FinallyIssueController.java:67) ~[main/:na]
... 50 common frames omitted
... ...
1
2
3
4
5
6
7
8
9
10
11
12
13
2
3
4
5
6
7
8
9
10
11
12
13
对于实现了AutoCloseable接口的资源型对象，可以使用ry-with-resource语法糖，但如果使用lombok的@Cleanup注解时，则还是会出现异常信息覆盖的问题。
public void useresourceright() throws Exception {
try (TestResource testResource = new TestResource()) {
testResource.read();
}
}
public void useresourceright2() throws Exception {
@Cleanup
TestResource testResource = new TestResource();
testResource.read();
}
1
2
3
4
5
6
7
8
9
10
11
12
2
3
4
5
6
7
8
9
10
11
12
public class TestResource implements AutoCloseable {
public void read() throws Exception {
throw new Exception("read error");
}
@Override
public void close() throws Exception {
throw new Exception("close error");
}
}
1
2
3
4
5
6
7
8
9
10
11
2
3
4
5
6
7
8
9
10
11
try-with-resource语法糖得到的日志
2022-01-13 00:55:38.154 ERROR 43529 --- [nio-8080-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet] : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.Exception: read error] with root cause
java.lang.Exception: read error
at com.liuyang19900520.bugs.l12.TestResource.read(TestResource.java:6) ~[main/:na]
at com.liuyang19900520.bugs.l12.FinallyIssueController.useresourceright(FinallyIssueController.java:27) ~[main/:na]
... ...
Suppressed: java.lang.Exception: close error
at com.liuyang19900520.bugs.l12.TestResource.close(TestResource.java:11) ~[main/:na]
at com.liuyang19900520.bugs.l12.FinallyIssueController.useresourceright(FinallyIssueController.java:26) ~[main/:na]
... 50 common frames omitted
1
2
3
4
5
6
7
8
9
10
2
3
4
5
6
7
8
9
10
@Cleanup注解得到的日志
2022-01-13 00:57:55.817 ERROR 43529 --- [nio-8080-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet] : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.Exception: close error] with root cause
java.lang.Exception: close error
at com.liuyang19900520.bugs.l12.TestResource.close(TestResource.java:11) ~[main/:na]
at com.liuyang19900520.bugs.l12.FinallyIssueController.useresourceright2(FinallyIssueController.java:34) ~[main/:na]
... ...
1
2
3
4
5
6
2
3
4
5
6
# 线程池中的异常
# 使用excute进行提交
public void execute() throws InterruptedException {
String prefix = "test";
ExecutorService threadPool = Executors.newFixedThreadPool(1, new ThreadFactoryBuilder()
.setNameFormat(prefix + "%d")
.setUncaughtExceptionHandler(
(thread, throwable) -> log.error("ThreadPool {} got exception", thread, throwable))
.build());
IntStream.rangeClosed(1, 10).forEach(i -> threadPool.execute(() -> {
if (i == 5) {
throw new RuntimeException("error");
}
log.info("I'm done : {}", i);
}));
threadPool.shutdown();
threadPool.awaitTermination(1, TimeUnit.HOURS);
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
当使用excute来提交的时候，我们会得到如下的日志。发现任务 6 开始运行在线程 test1。从线程名的改变可以知道因为异常的抛出老线程退出了，线程池只能重新创建一个线程。
2022-01-13 23:26:24.352 INFO 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 1
2022-01-13 23:26:24.353 INFO 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 2
2022-01-13 23:26:24.354 INFO 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 3
2022-01-13 23:26:24.354 INFO 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 4
2022-01-13 23:26:24.354 INFO 44729 --- [ test1] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 6
2022-01-13 23:26:24.354 INFO 44729 --- [ test1] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 7
2022-01-13 23:26:24.354 INFO 44729 --- [ test1] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 8
2022-01-13 23:26:24.354 INFO 44729 --- [ test1] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 9
2022-01-13 23:26:24.354 INFO 44729 --- [ test1] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 10
2022-01-13 23:26:24.358 ERROR 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : ThreadPool Thread[test0,5,main] got exception
java.lang.RuntimeException: error
at com.liuyang19900520.bugs.l12.ThreadPoolAndExceptionController.lambda$execute$2(ThreadPoolAndExceptionController.java:37) ~[main/:na]
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) ~[na:na]
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) ~[na:na]
at java.base/java.lang.Thread.run(Thread.java:834) ~[na:na]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
- 以 execute 方法提交到线程池的异步任务，最好在任务内部做好异常处理;
- 设置自定义的异常处理程序作为保底，比如在声明线程池时自定义线程池的未捕获异常处理程序；或者设置全局的默认未捕获异常处理程序。
# 使用submit提交
如果改为submit提交的话，我们会发现异常都会被吞到，日志中并没有输出异常。
public void submit() throws InterruptedException {
String prefix = "test";
ExecutorService threadPool = Executors.newFixedThreadPool(1,
new ThreadFactoryBuilder().setNameFormat(prefix + "%d").build());
IntStream.rangeClosed(1, 10).forEach(i -> threadPool.submit(() -> {
if (i == 5) {
throw new RuntimeException("error");
}
log.info("I'm done : {}", i);
}));
threadPool.shutdown();
threadPool.awaitTermination(1, TimeUnit.HOURS);
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
2022-01-14 00:09:40.458 INFO 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 1
2022-01-14 00:09:40.458 INFO 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 2
2022-01-14 00:09:40.459 INFO 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 3
2022-01-14 00:09:40.459 INFO 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 4
2022-01-14 00:09:40.459 INFO 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 6
2022-01-14 00:09:40.459 INFO 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 7
2022-01-14 00:09:40.459 INFO 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 8
2022-01-14 00:09:40.459 INFO 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 9
2022-01-14 00:09:40.459 INFO 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 10
1
2
3
4
5
6
7
8
9
2
3
4
5
6
7
8
9
既然是以 submit 方式来提交任务，那么我们应 该关心任务的执行结果，否则应该以 execute 来提交任务。 如果为了显示异常的话，需要调用我们的Future.get()方法。
public void submitRight() throws InterruptedException {
String prefix = "test";
ExecutorService threadPool = Executors.newFixedThreadPool(1,
new ThreadFactoryBuilder().setNameFormat(prefix + "%d").build());
List<Future> tasks = IntStream.rangeClosed(1, 10).mapToObj(i -> threadPool.submit(() -> {
if (i == 5) {
throw new RuntimeException("error");
}
log.info("I'm done : {}", i);
})).collect(Collectors.toList());
tasks.forEach(task -> {
try {
task.get();
} catch (Exception e) {
log.error("Got exception", e);
}
});
threadPool.shutdown();
threadPool.awaitTermination(1, TimeUnit.HOURS);
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
2022-01-14 00:16:29.590 INFO 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 1
2022-01-14 00:16:29.591 INFO 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 2
2022-01-14 00:16:29.591 INFO 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 3
2022-01-14 00:16:29.591 INFO 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 4
2022-01-14 00:16:29.591 INFO 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 6
2022-01-14 00:16:29.591 INFO 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 7
2022-01-14 00:16:29.591 INFO 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 8
2022-01-14 00:16:29.591 INFO 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 9
2022-01-14 00:16:29.591 INFO 44729 --- [ test0] c.l.b.l.ThreadPoolAndExceptionController : I'm done : 10
2022-01-14 00:16:29.595 ERROR 44729 --- [nio-8080-exec-1] c.l.b.l.ThreadPoolAndExceptionController : Got exception
java.util.concurrent.ExecutionException: java.lang.RuntimeException: error
at java.base/java.util.concurrent.FutureTask.report(FutureTask.java:122) ~[na:na]
at java.base/java.util.concurrent.FutureTask.get(FutureTask.java:191) ~[na:na]
... ...
at java.base/java.lang.Thread.run(Thread.java:834) ~[na:na]
Caused by: java.lang.RuntimeException: error
at com.liuyang19900520.bugs.l12.ThreadPoolAndExceptionController.lambda$submitRight$6(ThreadPoolAndExceptionController.java:72) ~[main/:na]
at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515) ~[na:na]
at java.base/java.util.concurrent.FutureTask.run$$$capture(FutureTask.java:264) ~[na:na]
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java) ~[na:na]
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) ~[na:na]
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) ~[na:na]
... 1 common frames omitted
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
确保正确处理了线程池中任务的异常，如果任务通过 execute 提交，那么出现异常会导致线程退出，大量的异常会导致线程重复创建引起性能问题，我们应该尽可能确保任务不出异常，同时设置默认的未捕获异常处理程序来兜底; 如果任务通过 submit 提交意味着 我们关心任务的执行结果，应该通过拿到的 Future 调用其 get 方法来获得任务运行结果和可能出现的异常，否则异常可能就被生吞了。
# 其他Java小知识
# Java线程池中submit和execute之间的区别
- 返回值
- submit()方法，可以提供Future < T > 类型的返回值。
- executor()方法，无返回值。
- 异常
- excute方法会抛出异常。
- sumbit方法不会抛出异常。除非你调用Future.get()。
- 参数
- excute入参Runnable
- submit入参可以为Callable