Situations where this happens:

• Convention property set at top of script, before plugin has been applied (easy to work around)
• Convention property defined in gradle.properties
• Convention property passed from the command-line with -P

I was interested in a fix of GRADDLE-1666 but this issue was marked as a duplicate of this one

... I encounter ClassCastExceptions in gradle-eclipse plugin, but I don't want to change the project under build (downloaded, with complex layout and tasks)

Caused by: java.lang.ClassCastException: java.math.BigDecimal cannot be cast to org.gradle.api.JavaVersion
at org.gradle.plugins.ide.eclipse.model.EclipseJdt_Decorated.getTargetCompatibility(Unknown Source)
at org.gradle.api.internal.BeanDynamicObject$MetaClassAdapter.getProperty(BeanDynamicObject.java:146)
at org.gradle.api.internal.BeanDynamicObject.getProperty(BeanDynamicObject.java:102)
at org.gradle.api.internal.CompositeDynamicObject.getProperty(CompositeDynamicObject.java:78)
at org.gradle.plugins.ide.eclipse.model.EclipseJdt_Decorated.getProperty(Unknown Source)
at org.gradle.plugins.ide.eclipse.GenerateEclipseJdt.configure(GenerateEclipseJdt.groovy:53)
at org.gradle.plugins.ide.eclipse.GenerateEclipseJdt.configure(GenerateEclipseJdt.groovy)
at org.gradle.plugins.ide.api.PropertiesGeneratorTask$1.configure(PropertiesGeneratorTask.java:37)
at org.gradle.plugins.ide.api.PropertiesGeneratorTask$1.configure(PropertiesGeneratorTask.java:31)
at org.gradle.plugins.ide.api.GeneratorTask.generate(GeneratorTask.java:75)
at org.gradle.api.internal.BeanDynamicObject$MetaClassAdapter.invokeMethod(BeanDynamicObject.java:216)
at org.gradle.api.internal.BeanDynamicObject.invokeMethod(BeanDynamicObject.java:122)
at org.gradle.api.internal.CompositeDynamicObject.invokeMethod(CompositeDynamicObject.java:147)
at org.gradle.plugins.ide.eclipse.GenerateEclipseJdt_Decorated.invokeMethod(Unknown Source)

However, I have noticed few things...

1) using in the project the verbose syntax "sourceCompatibility = JavaVersion.VERSION_1_6" can work (instead of "sourceCompatibility = 1.6")

2) there is a mismatch between field type and setter/getter type due to BeanDynamicObject _Generated class.
The following workaround works on gradle-eclipse plugin: changed type to "object", which is consistent with setter type... then called additional converter after getter for ensuring conversion / compile-type-check to type "JavaVersion"
Here is a git patch to apply

index b6f81f1..45b6e0e 100644
— a/subprojects/ide/src/main/groovy/org/gradle/plugins/ide/eclipse/model/EclipseJdt.groovy
+++ b/subprojects/ide/src/main/groovy/org/gradle/plugins/ide/eclipse/model/EclipseJdt.groovy
@@ -63,7 +63,8 @@ class EclipseJdt {

• <p>
• For example see docs for {@link EclipseJdt}
  */
  - JavaVersion sourceCompatibility
  +// JavaVersion sourceCompatibility
  + def sourceCompatibility
  
  void setSourceCompatibility(Object sourceCompatibility) {
  this.sourceCompatibility = JavaVersion.toVersion(sourceCompatibility)
  @@ -74,7 +75,8 @@ class EclipseJdt {
  * <p>
  * For example see docs for {@link EclipseJdt}

  */

• JavaVersion targetCompatibility
  +// JavaVersion targetCompatibility
  + def targetCompatibility

void setTargetCompatibility(Object targetCompatibility) {
this.targetCompatibility = JavaVersion.toVersion(targetCompatibility)

personally I think the string representation of the version should be also accepted every where, its much cleaner to store this type of properties in the gradle.properties file

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Thanks again for reporting this issue. We haven't heard back from you after our inquiry from November 15th. We are closing this issue now. Please create an issue on GitHub if you still feel passionate about getting it resolved.