test: add lambada test

mybatis-flex-core/src/main/java/com/mybatisflex/core/util/LambdaUtil.java

@@ -35,6 +35,10 @@ public class LambdaUtil {
     private static final Map<Class<?>, Class<?>> implClassMap = new ConcurrentHashMap<>();
     private static final Map<Class<?>, QueryColumn> queryColumnMap = new ConcurrentHashMap<>();
 
+    public static Map<Class<?>, String> getFieldNameMap() {
+        return fieldNameMap;
+    }
+
     public static <T> String getFieldName(LambdaGetter<T> getter) {
         return MapUtil.computeIfAbsent(fieldNameMap, getter.getClass(), aClass -> {
             SerializedLambda lambda = getSerializedLambda(getter);

mybatis-flex-core/src/test/java/com/mybatisflex/core/util/LambdaUtilTest.java

@@ -0,0 +1,18 @@
+package com.mybatisflex.core.util;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class LambdaUtilTest {
+
+    @Test
+    public void testIssue516() {
+
+        for (int i = 0; i < 100; i++) {
+            LambdaUtil.getFieldName(TestAccount::getName);
+        }
+
+        Assert.assertEquals(LambdaUtil.getFieldNameMap().size(), 1);
+        System.out.println("testIssue516");
+    }
+}

mybatis-flex-core/src/test/java/com/mybatisflex/core/util/TestAccount.java

@@ -0,0 +1,23 @@
+package com.mybatisflex.core.util;
+
+public class TestAccount {
+
+    private String name;
+    private int 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;
+    }
+}