导读
空指针异常
NPE 或许是编程语言中最常见的问题，被 Null 的发明者托尼·霍尔（Tony Hoare）称之为十亿美元的错误。在 Java 中并没有内置的处理 Null 值的语法，但仍然存在一些相对优雅的方式能够帮助我们的规避 NPE。
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使用 JSR-305/jetbrain 等注解
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NotNull -
Nullable
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用 Optional 处理链式调用
public class OptionalExample {public static void main(String[] args) {// 使用传统空值处理方式User user = getUser();String city = "DEFAULT";if (user != null && user.isValid()) {Address address = user.getAddress();if (adress != null) {city = adress.getCity();}}System.out.println(city);// 使用 Optional 的方式Optionaloptional = getUserOptional(); city = optional.filter(User::isValid).map(User::getAddress).map(Adress::getCity).orElse("DEFAULT")System.out.println(city);}public static User getUser() {return null;}public static OptionalgetUserOptional() {return Optional.empty();}public static class User {private Adress address;private boolean valid;}public static class Address {private String city;}}
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用 Objects.equals(a,b) 代替 a.equals(b)
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使用空对象模式
public class EmptyListExample {public static void main(String[] args) {ListString> listNullable = getListNullable();if (listNullable != null) {for (String s : listNullable) {System.out.println(s);}}ListString> listNotNull = getListNotNull();for (String s : listNotNull) {System.out.println(s);}}public static ListString> getListNullable() {return null;}public static ListString> getListNotNull() {return Collections.emptyList();}}
空策略
public class NullStrategyExample {private static final MapstrategyMap = new HashMap(); public static void handle(String strategy, String content) {findStrategy(strategy).handle(content);}private static Strategy findStrategy(String strategyKey) {return strategyMap.getOrDefault(strategyKey, new DoNothing());}public interface Strategy {void handle(String s);}// 当找不到对应策略时, 什么也不做public static class DoNothing implements Strategy {public void handle(String s) {}}}
对象转化
public class UserConverter {public static UserDTO toDTO(UserDO userDO) {UserDTO userDTO = new UserDTO();userDTO.setAge(userDO.getAge());// 问题 1: 自己赋值给自己userDTO.setName(userDTO.getName());return userDTO;}public static class UserDO {private String name;private Integer age;// 问题 2: 新增字段未赋值private String address;}public static class UserDTO {private String name;private Integer age;}}
反例2：
public class UserBeanCopyConvert {public UserDTO toDTO(UserDO userDO) {UserDTO userDTO = new UserDTO();// 用反射复制不同类型对象.// 1. 重构不友好, 当我要删除或修改 UserDO 的字段时, 无法得知该字段是否通过反射被其他字段依赖BeanUtils.copyProperties(userDO, userDTO);return userDTO;}}
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使用 Mapstruct
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未映射字段的处理策略，在编译期发现映射问题；
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复用工具，方便字段类型转化；
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生成 spring Component 注解，通过 spring 管理；
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等等其他特性；
(componentModel = "spring",unmappedSourcePolicy = ReportingPolicy.ERROR,unmappedTargetPolicy = ReportingPolicy.ERROR,// convert 逻辑依赖 DateUtil 做日期转化uses = DateUtil.class)public interface UserConvertor {UserDTO toUserDTO(UserDO userDO);class UserDO {private String name;private Integer age;//private String address;private Date birthDay;}class UserDTO {private String name;private Integer age;private String birthDay;}}public class DateUtil {public static String format(Date date) {SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");return simpleDateFormat.format(date);}}
使用示例：
public class UserService {private final UserDao userDao;private final UserCovertor userCovertor;public UserDTO getUser(String userId){UserDO userDO = userDao.getById(userId);return userCovertor.toUserDTO(userDO);}}
编译期校验：
(value = "org.mapstruct.ap.MappingProcessor",date = "2023-12-18T20:17:00+0800",comments = "version: 1.3.1.Final, compiler: javac, environment: Java 11.0.12 (GraalVM Community)")public class UserConvertorImpl implements UserConvertor {public UserDTO toUserDTO(UserDO userDO) {if ( userDO == null ) {return null;}UserDTO userDTO = new UserDTO();userDTO.setName( userDO.getName() );userDTO.setAge( userDO.getAge() );userDTO.setBirthDay( DateUtil.format( userDO.getBirthDay() ) );return userDTO;}}
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熟练使用线程安全类
public class ConcurrentHashMapExample {private Mapmap = new ConcurrentHashMap(); public void appendIfExists(String key, String suffix) {String value = map.get(key);if (value != null) {map.put(key, value + suffix);}}}
正例：
public class ConcurrentHashMapExample {private Mapmap = new ConcurrentHashMap(); public void append(String key, String suffix) {// 使用 computeIfPresent 原子操作map.computeIfPresent(key, (k, v) -> v + suffix);}}
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保证变更的原子性
public class NoAtomicDiamondParser {private volatile int start;private volatile int end;public NoAtomicDiamondParser() {Diamond.addListener("dataId", "groupId", new ManagerListenerAdapter() {public void receiveConfigInfo(String s) {JSONObject jsonObject = JSON.parseObject(s);start = jsonObject.getIntValue("start");end = jsonObject.getIntValue("end");}});}}public class MyController{private final NoAtomicDiamondParser noAtomicDiamondParser;public void handleRange(){// end 读取的旧值, start 读取的新值, start 可能大于 endint end = noAtomicDiamondParser.getEnd();int start = noAtomicDiamondParser.getStart();}}
正例：
public class AtomicDiamondParser {private volatile Range range;public AtomicDiamondParser() {Diamond.addListener("dataId", "groupId", new ManagerListenerAdapter() {public void receiveConfigInfo(String s) {range = JSON.parseObject(s, Range.class);}});}public static class Range {private int start;private int end;}}public class MyController {private final AtomicDiamondParser atomicDiamondParser;public void handleRange() {Range range = atomicDiamondParser.getRange();System.out.println(range.getStart());System.out.println(range.getEnd());}}
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使用不可变对象
public class AtomicDiamondParser {private volatile Range range;public AtomicDiamondParser() {Diamond.addListener("dataId", "groupId", new ManagerListenerAdapter() {public void receiveConfigInfo(String s) {JSONObject jsonObject = JSON.parseObject(s);int start = jsonObject.getIntValue("start");int end = jsonObject.getIntValue("end");range = new Range(start, end);}});}// lombok 注解会保证 Range 类的不变性public static class Range {private int start;private int end;}}
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正确性优先于性能
class Foo {// 缺少 volatile 关键字private Helper helper = null;public Helper getHelper() {if (helper == null)synchronized(this) {if (helper == null)helper = new Helper();}return helper;}}
在上述例子中，在 helper 字段上增加 volatile 关键字，能够在 java 5 及之后的版本中保证线程安全。
class Foo {private volatile Helper helper = null;public Helper getHelper() {if (helper == null)synchronized(this) {if (helper == null)helper = new Helper();}return helper;}}
正例3（推荐）：
class Foo {private Helper helper = null;public synchronized Helper getHelper() {if (helper == null)helper = new Helper();}return helper;}
并不严谨的 Diamond Parser：
/*** 省略异常处理等其他逻辑*/public class DiamondParser {// 缺少 volatile 关键字private Config config;public DiamondParser() {Diamond.addListener("dataId", "groupId", new ManagerListenerAdapter() {public void receiveConfigInfo(String s) {config = JSON.parseObject(s, Config.class);}});}public static class Config {private String name;}}
这种 Diamond 写法可能从来没有发生过线上问题，但这种写法也确实是不符合 JVM 线程安全原则。未来某一天你的代码跑在另一个 JVM 实现上，可能就有问题了。
线程池使用不当
public class ThreadPoolExample {// 没有任何限制的线程池, 使用起来很方便, 但当一波请求高峰到达时, 可能会创建大量线程, 导致系统崩溃private static Executor executor = Executors.newCachedThreadPool();}
反例 2：
public class StreamParallelExample {public ListString> batchQuery(ListString> ids){// 看上去很优雅, 但 ForkJoinPool 的队列是没有大小限制的, 并且线程数量很少, 如果 ids 列表很大可能导致 OOM// parallelStream 更适合计算密集型任务, 不要在任务中做远程调用return ids.parallelStream().map(this::queryFromRemote).collect(Collectors.toList());}private String queryFromRemote(String id){// 从远程查询}}
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手动创建线程池
public class ManualCreateThreadPool {// 手动创建资源有限的线程池private Executor executor = new ThreadPoolExecutor(10, 10, 1, TimeUnit.MINUTES, new ArrayBlockingQueue(1000),new ThreadFactoryBuilder().setNameFormat("work-%d").build());}
public class DuplicatedExceptionHandlerExample {private UserService userService;public User query(String id) {try {return userService.query(id);} catch (Exception e) {log.error("query error, userId: {}", id, e);return null;}}public User create(String id) {try {return userService.create(id);} catch (Exception e) {log.error("query error, userId: {}", id, e);return null;}}}
反例 2：
public class ExceptionShouldLogOrThrowExample {private UserService userService;public User query(String id) {try {return userService.query(id);} catch (Exception e) {// 异常被吞并, 问题被隐藏return null;}}public User create(String id) {try {return userService.create(id);} catch (Exception e) {// 堆栈丢失, 后续难以定位问题log.error("query error, userId: {}, error: {}", id,e.getMessage() );return null;}}}
反例 3：
public class OpenAPIService {public void handle(){// HSF 服务对外抛出 client 中未定义的异常, 调用方反序列化失败throw new InternalSystemException("");}}
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通过 AOP 统一异常处理
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避免未知异常抛给调用方, 将未知异常转为 Result 或者通用异常类型 -
统一异常日志的打印和监控
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处理 Checked Exception
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Try catch 线程逻辑
public class ThreadNotTryCatch {private final ExecutorService executorService;public void handle() {executorService.submit(new Runnable() {public void run() {// 未捕获异常, 线程直接退出, 异常信息丢失remoteInvoke();}});}}
正例：
4jpublic class ThreadNotTryCatch {private final ExecutorService executorService;public void handle() {executorService.submit(new Runnable() {public void run() {try {remoteInvoke();} catch (Exception e) {log.error("handle failed", e);}}});}}
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特殊异常的处理
public class InterruptedExceptionExample {private ExecutorService executorService = Executors.newSingleThreadExecutor();public void handleWithTimeout() throws InterruptedException {Future> future = executorService.submit(() -> {try {// sleep 模拟处理逻辑Thread.sleep(1000);} catch (InterruptedException e) {System.out.println("interrupted");}System.out.println("continue task");// 异常被忽略, 继续处理});// 等待任务结果, 如果超过 500ms 则中断Thread.sleep(500);if (!future.isDone()) {System.out.println("cancel");future.cancel(true);}}}
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避免 catch Error
public class ErrorExample {private Date start;private Date end;public synchronized void update(long start, long end) {if (start > end) {throw new IllegalArgumentException("start after end");}this.start = new Date(start);// 如果 new Date(end) 发生 OOM, start 有可能大于 endthis.end = new Date(end);}}
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反例 1: SpringContext 作为静态变量
public class SpringContextUtils {private static ApplicationContext applicationContext;public SpringContextUtils(ApplicationContext context) {applicationContext = context;}}public class UserController {public void handle(){MyService bean = SpringContextUtils.getApplicationContext().getBean(MyService.class);}}
反例 2: Switch 在 Spring Bean 中注册, 但通过静态方式读取
public class SwitchConfig {public void init() {SwitchManager.register("appName", MySwitch.class);}public static class MySwitch {(des = "config", level = Switch.Level.p1)public static String config;}}public class UserController{public String getConfig(){// UserController 和 SwitchConfig 类没有依赖关系, MySwitch.config 可能还没有初始化return MySwitch.config;}}
通过 SpringBeanFactory 保证初始化顺序：
public class PreInitializer implements BeanFactoryPostProcessor, PriorityOrdered {public int getOrder() {return Ordered.HIGHEST_PRECEDENCE;}public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {try {SwitchManager.init(应用名, 开关类.class);} catch (SwitchCenterException e) {// 此处抛错最好阻断程序启动，避免开关读不到持久值引发问题} catch (SwitchCenterError e) {System.exit(1);}}}
public class SpringContextUtilPostProcessor implements BeanFactoryPostProcessor, PriorityOrdered, ApplicationContextAware {private ApplicationContext applicationContext;public int getOrder() {return Ordered.HIGHEST_PRECEDENCE;}public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory)throws BeansException {SpringContextUtils.setApplicationContext(applicationContext);}public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {this.applicationContext = applicationContext;}}
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LoadingCache 代替全局 Map
public class MetaInfoManager {// 对于少量的元数据来说, 放到内存中似乎并无大碍, 但如果后续元数据量增大, 则大量对象则内存中无法释放, 导致内存泄漏private MapString, MetaInfo> cache = new HashMap();public MetaInfo getMetaInfo(String id) {return cache.computeIfAbsent(id, k -> loadFromRemote(id));}private LoadingCacheString, MetaInfo> loadingCache = CacheBuilder.newBuilder()// loadingCache 设置最大 size 或者过期时间, 能够限制缓存条目的数量.maximumSize(1000).build(new CacheLoaderString, MetaInfo>() {public MetaInfo load(String key) throws Exception {return loadFromRemote(key);}});public MetaInfo getMetaInfoFromLoadingCache(String id) {return loadingCache.getUnchecked(id);}private MetaInfo loadFromRemote(String id) {return null;}public static class MetaInfo {private String id;private String name;}}
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谨慎使用运行时类生成技术
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使用 Try With Resource
public class TryWithResourceExample {public static void main(String[] args) throws IOException {try (InputStream in = Files.newInputStream(Paths.get(""))) {// read}}}
public class URLExample {public void handle(URL a, URL b) {if (Objects.equals(a, b)) {}}}
反例 2：
public class URLMapExample {private static final MapurlObjectMap = new HashMap(); }
循环远程调用：
public class HSFLoopInvokeExample {@HSFConsumerprivate UserService userService;public ListbatchQuery(List ids){ // 使用批量接口或者限制批量大小return ids.stream().map(userService::getUser).collect(Collectors.toList());}}
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了解常见性能指标&瓶颈
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使用专业性能测试工具估性能
public class ManualPerformanceTest {public void testPerformance() {long start = System.currentTimeMillis();for (int i = 0; i 1000; i++) {// 这里 mutiply 没有任何副作用, 有可能被优化之后被干掉mutiply(10, 10);}System.out.println("avg rt: " + (System.currentTimeMillis() - start) / 1000);}private int mutiply(int a, int b) {return a * b;}}
正例:
(iterations = 5, time = 1, timeUnit = TimeUnit.SECONDS)(iterations = 5, time = 1, timeUnit = TimeUnit.SECONDS)(3)(Mode.AverageTime)(TimeUnit.NANOSECONDS)public class JMHExample {public void testPerformance(Blackhole bh) {bh.consume(mutiply(10, 10));}private int mutiply(int a, int b) {return a * b;}}
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注意事务注解失效的场景
public class TransactionNotWork {public void doTheThing() {actuallyDoTheThing();}public void actuallyDoTheThing() {}}
参考资料：
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Null：价值 10 亿美元的错误: https://www.infoq.cn/article/uyyos0vgetwcgmo1ph07 -
双重检查锁失效声明: https://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html -
每个程序员都应该知道的延迟数字: https://colin-scott.github.io/personal_website/research/interactive_latency.html
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