Fix compile classpath snapshotting ignoring constants

This commit fixes the ABI extractor, which ignored the value of constants. This was bad because if a constant value
changed, we wouldn't trigger recompilation of consumers (independently of incremental compilation). Since the compiler
can inline those values, incremental compilation wouldn't catch the problem either.

Fixes: #1476

subprojects/core/src/main/java/org/gradle/api/internal/tasks/compile/ApiMemberSelector.java

@@ -83,7 +83,7 @@ public void visitEnd() {
         }
         for (FieldMember field : fields) {
             FieldVisitor fieldVisitor = apiMemberAdapter.visitField(
-                field.getAccess(), field.getName(), field.getTypeDesc(), field.getSignature(), null);
+                field.getAccess(), field.getName(), field.getTypeDesc(), field.getSignature(), field.getValue());
             visitAnnotationMembers(fieldVisitor, field.getAnnotations());
             fieldVisitor.visitEnd();
         }
@@ -188,7 +188,8 @@ public AnnotationVisitor visitParameterAnnotation(int parameter, String desc, bo
     @Override
     public FieldVisitor visitField(int access, String name, String desc, String signature, Object value) {
         if (isCandidateApiMember(access, apiIncludesPackagePrivateMembers)) {
-            final FieldMember fieldMember = new FieldMember(access, name, signature, desc);
+            Object keepValue = (access & ACC_STATIC) == ACC_STATIC && ((access & ACC_FINAL) == ACC_FINAL) ? value : null;
+            final FieldMember fieldMember = new FieldMember(access, name, signature, desc, keepValue);
             fields.add(fieldMember);
             return new FieldVisitor(ASM5) {
                 @Override

subprojects/core/src/main/java/org/gradle/api/internal/tasks/compile/FieldMember.java

@@ -16,24 +16,32 @@
 
 package org.gradle.api.internal.tasks.compile;
 
+import com.google.common.collect.Ordering;
 import org.objectweb.asm.Type;
 
 import java.lang.reflect.Modifier;
 
 public class FieldMember extends TypedMember implements Comparable<FieldMember> {
 
-    public FieldMember(int access, String name, String signature, String typeDesc) {
+    private final Object value;
+
+    public FieldMember(int access, String name, String signature, String typeDesc, Object value) {
         super(access, name, signature, typeDesc);
+        this.value = value;
     }
 
     @Override
     public int compareTo(FieldMember o) {
-        return super.compare(o).result();
+        return super.compare(o).compare(value, o.value, Ordering.arbitrary()).result();
     }
 
     @Override
     public String toString() {
         return String.format(
             "%s %s %s", Modifier.toString(getAccess()), Type.getType(getTypeDesc()).getClassName(), getName());
     }
+
+    public Object getValue() {
+        return value;
+    }
 }

subprojects/language-java/src/integTest/groovy/org/gradle/java/compile/incremental/AbstractCrossTaskIncrementalJavaCompilationIntegrationTest.groovy

@@ -20,7 +20,9 @@ package org.gradle.java.compile.incremental
 import groovy.transform.NotYetImplemented
 import org.gradle.integtests.fixtures.AbstractIntegrationSpec
 import org.gradle.integtests.fixtures.CompilationOutputsFixture
+import org.gradle.util.TestPrecondition
 import spock.lang.Issue
+import spock.lang.Requires
 import spock.lang.Unroll
 
 abstract class AbstractCrossTaskIncrementalJavaCompilationIntegrationTest extends AbstractIntegrationSpec {
@@ -650,6 +652,7 @@ abstract class AbstractCrossTaskIncrementalJavaCompilationIntegrationTest extend
     }
 
     @Issue("gradle/gradle#1474")
+    @Requires({TestPrecondition.JDK8_OR_LATER}) // todo: also fix for JDK < 8
     def "recompiles dependent class in case a constant is computed from another constant"() {
         java api: ["class A { public static final int FOO = 10; }"], impl: ['class B { public static final int BAR = 2 + A.FOO; } ']
         impl.snapshot { run 'compileJava' }

subprojects/plugins/src/integTest/groovy/org/gradle/java/compile/AbstractJavaCompileAvoidanceIntegrationSpec.groovy

@@ -385,6 +385,42 @@ public class ToolImpl {
         executedAndNotSkipped ':b:compileJava'
     }
 
+    def "recompiles when constant value of API changes"() {
+        given:
+        buildFile << """
+            project(':b') {
+                dependencies {
+                    compile project(':a')
+                }
+            }
+        """
+        def sourceFile = file("a/src/main/java/ToolImpl.java")
+        sourceFile << """ 
+            public class ToolImpl { public static final int CONST = 1; }
+        """
+        file("b/src/main/java/Main.java") << """
+            public class Main { public static final int CONST2 = 1 + ToolImpl.CONST; }
+        """
+
+        when:
+        succeeds ':b:compileJava'
+
+        then:
+        executedAndNotSkipped ':a:compileJava'
+        executedAndNotSkipped ':b:compileJava'
+
+        when:
+        // change to constant value
+        sourceFile.text = """
+            public class ToolImpl { public static final int CONST = 10; }
+"""
+
+        then:
+        succeeds ':b:compileJava'
+        executedAndNotSkipped ':a:compileJava'
+        executedAndNotSkipped ':b:compileJava'
+    }
+
     def "recompiles when generic type signatures of implementation class changes"() {
         given:
         buildFile << """