How automatically fill object path to avoid NPE in groovy? - groovy

Question

I have such code:
a.b = null
a.b.c = 10 // here is NPE should be thrown
Is there way to avoid NPE by some automatic interception and initialization of b with new instance value?
In common case the object path may be long like this: a.b.c.d.e.f = 10
and any part of path may be met a null value

Answer 1

If you don't want to insert safe-navigation operators like a?.b?.c?.d?.e?.f = 10 you can use a macro to do this "automatically":
// I have not tried with assignment so you may need to fill a gap in the macro method
safe(a.b.c.d.e.f = 10)
// I have tested like this:
#Test
void testSafeOne() {
def objExp = null
def hashCode = safe(objExp.hashCode())
assert safe(hashCode.toString()) == null
}
#Test
void testSafeMany() {
def objExp = null
assert safe(objExp.hashCode().toString()) == null
}
#Test
void testSafeAttr() {
Integer number = null
assert safe(number.#MAX_VALUE.abs()) == null
}
#Test
void testSafeAssign() {
// TODO
}
You need to register this class through the extension mechanism:
package macros
import org.codehaus.groovy.ast.*
import org.codehaus.groovy.ast.expr.*
import org.codehaus.groovy.macro.runtime.Macro
import org.codehaus.groovy.macro.runtime.MacroContext
import static org.codehaus.groovy.ast.ClassHelper.*
import static org.codehaus.groovy.ast.tools.GeneralUtils.*
import static org.apache.groovy.ast.tools.ExpressionUtils.isThisExpression
class SafeMacroMethods {
/**
* macro version of: <code>objectExpr.?methodName()</code>
*/
#Macro
static Expression safe(MacroContext context, MethodCallExpression expression) {
if (isImplicitThis(context)) {
expression.tap { safe = true; safe(context, objectExpression) }
}
}
/**
* macro version of: <code>objectExpr.?propertyName</code>
*/
#Macro
static Expression safe(MacroContext context, PropertyExpression expression) {
if (isImplicitThis(context)) {
expression.tap {
if (getClass() == PropertyExpression) {
it.#safe = true
} else { // AttributeExpression, etc.
def field = PropertyExpression.getDeclaredField('safe')
field.accessible = true
field.set(it, true)
}
safe(context, objectExpression)
}
}
}
/**
* macro version of: <code>targetExpr ?= sourceExpr</code>
*/
#Macro
static Expression safe(MacroContext context, BinaryExpression expression) {
if (isImplicitThis(context)) {
expression.tap { safe = true; safe(context, leftExpression); safe(context, rightExpression) }
}
}
protected
static Expression safe(MacroContext context, Expression expression) {
expression
}
private static boolean isImplicitThis(MacroContext context) {
context.call.with {
isThisExpression(objectExpression) && isImplicitThis()
}
}
}