空指针异常是导致java程序运行中断最常见的原因，相信每个程序猿都碰见过，也就是NullPointException，我们通常简称为NPE，本文告诉大家如何优雅避免NPE。

1.数据准备

package npe;/*** @author 百里*/
public class User {private String name;private int age;private Address address;public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}public Address getAddress() {return address;}public void setAddress(Address address) {this.address = address;}public User(){}public User(String name, int age, Address address) {this.name = name;this.age = age;this.address = address;}@Overridepublic String toString() {return "User{" +"name='" + name + '\'' +", age=" + age +", address=" + address +'}';}
}

package npe;/*** @author 百里*/
public class Address {private String street;private String city;private String country;public Address(){}public Address(String street, String city, String country) {this.street = street;this.city = city;this.country = country;}// getters and setterspublic String getStreet() {return street;}public void setStreet(String street) {this.street = street;}public String getCity() {return city;}public void setCity(String city) {this.city = city;}public String getCountry() {return country;}public void setCountry(String country) {this.country = country;}@Overridepublic String toString() {return "Address{" +"street='" + street + '\'' +", city='" + city + '\'' +", country='" + country + '\'' +'}';}
}

UML类关系图：

2.实战：获取用户所在的城市

2.1.直接获取；容易出现空指针异常。

/*** 获取人员所在的城市* @author 百里*/
public class BaiLiNpeDemo {public static void main(String[] args) {Address myAddress = new Address();User myUser = new User("John Doe", 35, myAddress);String city = myUser.getAddress().getCity().trim();System.out.println(city);}
}

2.2.使用if-else判断；避免了出现空指针的问题，但是代码结构层次嵌套多，不美观

/*** 使用if进行判断* @author 百里*/
public class BaiLiSimpleNpeDemo {public static void main(String[] args) {Address myAddress = new Address();User myUser = new User("John Doe", 35, myAddress);if (myUser != null) {Address address = myUser.getAddress();if (address != null) {String city = address.getCity();if (city != null  && !"".equals(city)) {System.out.println("使用if判断字符串：" + "一键三连");}}}}
}

2.3.使用工具类美化一下if判断代码

/*** 使用工具类* @author 百里*/
public class BaiLiUtilsNpeDemo {public static void main(String[] args) {Address myAddress = new Address("123 Main St", " Austin ", "CA");User myUser = new User("John Doe", 35, myAddress);//针对对象与字符串if (!ObjectUtils.isEmpty(myUser)) {Address address = myUser.getAddress();if (!ObjectUtils.isEmpty(address)) {String city = address.getCity();if (!StringUtils.isEmpty(city)) {System.out.println("使用StringUtils工具类判断字符串：" + "一键三连");}}}//针对数组使用工具类ArrayList<User> users = new ArrayList<>();users.add(myUser);if (!CollectionUtils.isEmpty(users)) {System.out.println("使用CollectionUtils工具类判断数组对象：" + "一键三连");}}
}

2.4.使用Optional解决了层次多的问题也避免了空指针的问题，当我们配合使用orElse时，会先执行orElse方法，然后执行逻辑代码，不管是否出现了空指针。

/*** 使用Optional* @author 百里*/
public class BaiLiOptionalNpeDemo {public static void main(String[] args) {Address myAddress = new Address();User myUser = new User("John Doe", 35, myAddress);System.out.println("使用Optional判断 + orElse:" +Optional.ofNullable(myUser).map(User::getAddress).map(Address::getCity).map(String::trim).orElse(getDefaultCity()));}//初始化城市public static String getDefaultCity() {System.out.println("初始化默认城市");return null;}
}

2.5.使用断言处理接口入参，检查假设和前置条件是否满足，以及检查空值情况，提前捕获空指针异常并进行处理

import org.springframework.util.Assert;/*** 接口参数校验* @author 百里*/
public class BaiLiAssertNpeDemo {public static void main(String[] args) {Address myAddress = new Address("123 Main St", " Austin ", "CA");User user = new User("John Doe", 35, myAddress);getUserCity(user);getUserCity(null);}public static void getUserCity(User user){Assert.notNull(user,"user is null");Address address = user.getAddress();Assert.notNull(address,"address is null");String city = address.getCity();System.out.println(city);}
}

2.6.使用@Nullable注解，标识变量或方法参数和返回值是否可以为 null，以便在编译期或开发工具中提示可能的 NullPointerException 风险

/*** 使用注解 @Nullable* @author 百里*/
public class BaiLiNonNullDemo {public static void printString(@Nullable String str) {System.out.println(str.toString());}@Nullablepublic static String getString() {return null;}public static void main(String[] args) {String str = null;printString(str);getString().toString();User user = new User();user.getAddress().getCity();}
}

2.7.额外补充

JDK17优化了空指针异常信息(Helpful NullPointerExceptions)
通过精确描述哪个变量为空来提高JVM生成的空指针异常信息的可用性。
即，以前的空指针异常信息不会告诉你具体是哪个对象为null，当运行的语句是对一个嵌套结构的对象做连续的方法调用(如"a.getb().getc().xxx()")时，就需要进一步分析或调试才能判断出谁是null。而该特性加入以后则直接在异常信息中说明值为null的对象是哪个。

/*** @author 百里*/
public class BaiLiNpeDemo {public static void main(String[] args) {Address myAddress = new Address("123 Main St", null, "CA");User myUser = new User("John Doe", 35, myAddress);System.out.println(myUser.getAddress().getCity().trim());}
}

执行结果：

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "String.trim()" because the return value of "npe.Address.getCity()" is nullat npe.BaiLiNpeDemo.main(BaiLiNpeDemo.java:16)