问题
I have this class hierarchy
StudentClass .java
public class StudentClass {
private List<Student> studentList;
public List<Student> getStudentList() {
return studentList;
}
public void setStudentList(List<Student> studentList) {
this.studentList = studentList;
}
}
Student.java
public class Student {
private Child child;
private int studAge;
public Student(Child child, int studAge) {
this.child = child;
this.studAge = studAge;
}
public Child getChild() {
return child;
}
public void setChild(Child child) {
this.child = child;
}
public int getStudAge() {
return studAge;
}
public void setStudAge(int studAge) {
this.studAge = studAge;
}
}
Child.java
public class Child {
private String name;
private int age;
public Child(String name, int age) {
this.name = name;
this.age = age;
}
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;
}
}
Main class
Through some logic i build this expression to execute through MVEL. This piece of code works fine when child2 is null, but when child1 is null it gives below error.
This is a valid scenario, and it is just a replica of my entities from my application. Child might come as null inside Parent.
Issue only comes when object is null at index 0 , rest all indexes it works fine, even if at index 1 it is null, and index 0 fails if condition, then it will execute at index 2
public class MvelTest {
public static void main(String args[]) throws Exception {
String s = "if(contextObjectStudentClass.?studentList != null ){ foreach ( loopVariable0 : contextObjectStudentClass.?studentList){if ( loopVariable0.?child.?age==21 ){return loopVariable0.?child.?name ;}}}return null ;";
Child child2 = new Child("ankur", 23);
Child child1 = null;
Child child3 = new Child("ankurs", 21);
Student s1 = new Student(child1, 21);
Student s2 = new Student(child2, 23);
Student s3 = new Student(child3, 27);
List<Student> studentList = new ArrayList<Student>();
studentList.add(s1);
studentList.add(s2);
studentList.add(s3);
StudentClass class1 = new StudentClass();
class1.setStudentList(studentList);
Map map = new HashMap();
map.put("contextObjectStudentClass", class1);
System.out.println(MVEL.eval(s, map));
}
}
Exception
Exception in thread "main" java.lang.RuntimeException: cannot invoke getter: getChild (see trace)
at org.mvel2.optimizers.impl.refl.nodes.GetterAccessor.getValue(GetterAccessor.java:70)
at org.mvel2.optimizers.impl.refl.nodes.VariableAccessor.getValue(VariableAccessor.java:37)
at org.mvel2.optimizers.dynamic.DynamicGetAccessor.getValue(DynamicGetAccessor.java:73)
at org.mvel2.ast.ASTNode.getReducedValueAccelerated(ASTNode.java:108)
at org.mvel2.ast.BinaryOperation.getReducedValueAccelerated(BinaryOperation.java:114)
at org.mvel2.compiler.ExecutableAccessor.getValue(ExecutableAccessor.java:38)
at org.mvel2.ast.IfNode.getReducedValueAccelerated(IfNode.java:73)
at org.mvel2.compiler.ExecutableAccessor.getValue(ExecutableAccessor.java:38)
at org.mvel2.ast.ForEachNode.getReducedValue(ForEachNode.java:136)
at org.mvel2.MVELInterpretedRuntime.parseAndExecuteInterpreted(MVELInterpretedRuntime.java:106)
at org.mvel2.MVELInterpretedRuntime.parse(MVELInterpretedRuntime.java:49)
at org.mvel2.MVEL.eval(MVEL.java:408)
at org.mvel2.ast.IfNode.getReducedValue(IfNode.java:89)
at org.mvel2.MVELInterpretedRuntime.parseAndExecuteInterpreted(MVELInterpretedRuntime.java:106)
at org.mvel2.MVELInterpretedRuntime.parse(MVELInterpretedRuntime.java:49)
at org.mvel2.MVEL.eval(MVEL.java:165)
at com.nucleus.rules.service.MvelTest.main(MvelTest.java:34)
Caused by: java.lang.NullPointerException
at org.mvel2.optimizers.impl.refl.nodes.NullSafe$1.getValue(NullSafe.java:39)
at org.mvel2.optimizers.impl.refl.nodes.NullSafe.getValue(NullSafe.java:54)
at org.mvel2.optimizers.impl.refl.nodes.GetterAccessor.getValue(GetterAccessor.java:40)
... 16 more
回答1:
Its work for me in version 2.0. I have tried even null in 2 or 3 position its throwing exception
I have attached exception
Exception in thread "main" [Error: cannot invoke getter: getChild [declr.class: Student; act.class: Student]]
[Near : {... ame ;}}} else { return "Noo" } ....}]
[Line: 1, Column: 218]
at org.mvel.MVELInterpretedRuntime.parseAndExecuteInterpreted(MVELInterpretedRuntime.java:156)
at org.mvel.MVELInterpretedRuntime.parse(MVELInterpretedRuntime.java:54)
at org.mvel.MVEL.eval(MVEL.java:124)
at MvelTest.main(MvelTest.java:33)
Caused by: [Error: cannot invoke getter: getChild [declr.class: Student; act.class: Student]]
[Near : {... loopVariable0.?child.?name ;} ....}]
[Line: 1, Column: 143]
at org.mvel.MVELInterpretedRuntime.parseAndExecuteInterpreted(MVELInterpretedRuntime.java:156)
at org.mvel.MVELInterpretedRuntime.parse(MVELInterpretedRuntime.java:54)
at org.mvel.MVEL.eval(MVEL.java:107)
at org.mvel.ast.IfNode.getReducedValue(IfNode.java:64)
at org.mvel.MVELInterpretedRuntime.parseAndExecuteInterpreted(MVELInterpretedRuntime.java:106)
... 3 more
Caused by: [Error: cannot invoke getter: getChild [declr.class: Student; act.class: Student]]
[Near : {... Unknown ....}]
at org.mvel.optimizers.impl.refl.GetterAccessor.getValue(GetterAccessor.java:51)
at org.mvel.optimizers.impl.refl.VariableAccessor.getValue(VariableAccessor.java:38)
at org.mvel.optimizers.dynamic.DynamicGetAccessor.getValue(DynamicGetAccessor.java:44)
at org.mvel.ast.ASTNode.getReducedValueAccelerated(ASTNode.java:97)
at org.mvel.ast.BinaryOperation.getReducedValueAccelerated(BinaryOperation.java:64)
at org.mvel.compiler.ExecutableAccessor.getValue(ExecutableAccessor.java:46)
at org.mvel.ast.IfNode.getReducedValueAccelerated(IfNode.java:48)
at org.mvel.compiler.ExecutableAccessor.getValue(ExecutableAccessor.java:46)
at org.mvel.ast.ForEachNode.getReducedValue(ForEachNode.java:148)
at org.mvel.MVELInterpretedRuntime.parseAndExecuteInterpreted(MVELInterpretedRuntime.java:106)
... 7 more
Caused by: [Error: cannot invoke getter: getAge [declr.class: Child; act.class: null]]
[Near : {... Unknown ....}]
at org.mvel.optimizers.impl.refl.GetterAccessor.getValue(GetterAccessor.java:51)
at org.mvel.optimizers.impl.refl.GetterAccessor.getValue(GetterAccessor.java:38)
... 16 more
Caused by: java.lang.NullPointerException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at org.mvel.optimizers.impl.refl.GetterAccessor.getValue(GetterAccessor.java:41)
... 17 more
来源：https://stackoverflow.com/questions/26839705/mvel-iterate-a-list