Java 简明教程
Java - Null Pointer Exception
在 Java 14 中，引入了一个新功能，称为 NullPointerException，它非常有用。当时这只是一个预览功能，后来成为了 JDK 的标准部分。这种增强的动机是使用更多详细信息丰富 NullPointerException，因为传统的 NullPointerException 只提供了文件名称、方法和发生 NullPointerException 的行号的详细信息。
In Java 14, a new feature was introduced as helpful NullPointerException. It was a preview feature then and later became the standard part of the JDK. The motivation behind this enhancement was to enrich the NullPointerException with more details as a traditional NullPointerException only gave details of filename, method, and the line number where a NullPointerException occurred.
Traditional Null Pointer Exception in Java
在 Java 14 NullPointerException 增强之前，以下代码将显示一条如下所示的 NullPointerException 消息：
Before Java 14 NullPointerException enhancement, following code will show a NullPointerException message as shown below:
// Create an employee without department
Employee emp = new Employee(1,"Robert",null);
// get the name of the dept
// as name is null, this code will throw a null pointer exception
String dept = emp.getDept().getName();如果使用此代码片段进行运行，将生成类似的结果 −
If this code snippet is used to run, this will produce similar result −
Exception in thread "main" java.lang.NullPointerException
at com.tutorialspoint.Tester.main(Tester.java:10)New Informative Null Pointer Exception in Java
在调试过程中，此类错误消息没有用处。并且随着嵌套级别增加，检查哪个字段为 null 导致了此问题变得更加困难。Java 14 满足了此需求。在这样的情况下，它提供了一个选项，以返回以下更友好的消息：
During debugging, such a error message is not useful. And with multiple nesting level, it becomes more difficult to check which field being null is causing the issue. Java 14 catered to this need. It provided an option to return a more helpful message like below in such case:
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "com.tutorialspoint.Department.getName()" because the return value of "com.tutorialspoint.Employee.getDept()" is null
at com.tutorialspoint.Tester.main(Tester.java:10)在这里我们可以看到，正在打印更友好的信息消息来帮助调试 getDept() 为 null 的问题。为了在 Java 14 中启用此功能，我们必须使用以下标志来运行 java 代码。
Here we can see, that a more information message is getting printed to assist in debugging the issue that getDept() is null. In order to enable this feature in Java 14, we’ve to run the java code using following flag.
-XX:+ShowCodeDetailsInExceptionMessages现在使用 Java 20，不再需要此标志。
Now with Java 20, this flag is not needed anymore.
Helpful NullPointerException for Null Objects
在这个示例中，我们展示了以上学习到的概念。它是一个很有用的特性，用于调试由 null 引用导致的问题。在这里，我们创建了两个类 Employee 和 Department。在 main 方法中，我们创建了 Employee 类的对象，但是将 department 传递为 null。然后，我们尝试获取员工的部门名称，这会产生 NullPointerException。在输出中，我们可以看到 JVM 引发的信息丰富的空指针异常。
In this example, we’re showcasing the above learnt concept. It is a very useful feature to debug the issues caused due to null references. Here we’ve created two classes Employee and Department. In main method, we’ve created the object of Employee class but department is passed as null. Then we’ve tried to get the department name of the employee which results in a NullPointerException. In output we can see the informative null pointer exception raised by the JVM.
Example
package com.tutorialspoint;
import java.util.List;
public class Tester {
public static void main(String[] args) {
// declare an employee without department
Employee emp = new Employee(1,"Robert",null);
// get department name of null department will
// throw a NullPointerException
String dept = emp.getDept().getName();
System.out.println(dept);
}
}
class Employee {
int id;
String name;
Department dept;
Employee (int id, String name, Department dept){
this.id = id;
this.name = name;
this.dept = dept;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Department getDept() {
return dept;
}
public void setDept(Department dept) {
this.dept = dept;
}
}
class Department {
int id;
String name;
List<Employee> employees;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<Employee> getEmployees() {
return employees;
}
public void setEmployees(List<Employee> employees) {
this.employees = employees;
}
}Output
让我们编译并运行上述程序，这将生成以下结果 −
Let us compile and run the above program, this will produce the following result −
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "com.tutorialspoint.Department.getName()" because the return value of "com.tutorialspoint.Employee.getDept()" is null
at com.tutorialspoint.Tester.main(Tester.java:10)Informative NullPointerException - Local Null Variable
与对象引用类似，局部 variable 可能为 null。在 Java 14 中，如果使用了 null 局部变量，那么 JVM 会打印变量的名称为 <local>，如下所示：
Similar to object reference, local variable can be null. In Java 14, if a null local variable is used, then JVM prints the name of variable as <local> as shown below:
String name = null;
System.out.println("Length: " + name.length() );如果使用此代码片段进行运行，将生成类似的结果 −
If this code snippet is used to run, this will produce similar result −
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "String.length()" because "<local1>" is null
at com.tutorialspoint.Tester.main(Tester.java:8)为了打印局部变量的名称，我们在运行程序时必须使用以下标志。
In order to print the name of the local variable, we had to use following flag while running the program.
-g使用此标志会产生以下输出。
Using this flag results in following output.
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "String.length()" because "name" is null
at com.tutorialspoint.Tester.main(Tester.java:8)在 Java 20 及更高版本中，不再需要此标志。请参见以下完整示例：
In Java 20 onwards, this flag is not needed anymore. See the complete example below:
Helpful NullPointerException for Null Local Variable
在这个示例中，我们展示了以上学习到的概念，以调试 null 局部引用。在这里，我们创建了一个局部变量 name，并且尝试获取 null string 变量的长度，这会导致 NullPointerException。在输出中，我们可以看到 JVM 引发的带有 null 变量实际名称的信息丰富的空指针异常。
In this example, we’re showcasing the above learnt concept to debug null local references. Here we’ve created a local variable name and we’re trying to the get the length of null string variable which will cause the NullPointerException. In output we can see the informative null pointer exception raised by the JVM with the actual name of the null variable.