MybatisPlus LambdaQueryWrapper类的实现原理

### jdk8 lambda是实现原理

1.  定义一个函数式接口Func

```java
@FunctionalInterface
interface Func extends Serializable {
    String func(Object obj);
}
```

2.  编写一段代码

```java
Func func=Object::toString;
1
```

等价于,但上面最终可以获取toString方法名，下面只能获取到编译器生成的方法名

```java
Func func=o->o.toString()
```

3.  运行代码，jvm加上`-Djdk.internal.lambda.dumpProxyClasses=(Path)`，可以看`java.lang.invoke.InnerClassLambdaMetafactory`类

![](/media/202311/2023-11-15_151956_9802020.3535549527725139.png)

```java
public class MailMicroServiceApplication {
    public static void main(String[] args) {
        Func func=Object::toString;
        func.func(new Object());
    }
}
```

4.运行完成后会生成

![](/media/202311/2023-11-15_152036_8924330.22298206707773938.png)

5.idea 工具反编译后

```java
package top.hunfan.mail;

import java.lang.invoke.SerializedLambda;
import java.lang.invoke.LambdaForm.Hidden;

// $FF: synthetic class
final class MailMicroServiceApplication$$Lambda$85 implements Func {
    private MailMicroServiceApplication$$Lambda$85() {
    }

@Hidden
    public String func(Object var1) {
        return var1.toString();
    }

private final Object writeReplace() {
        return new SerializedLambda(MailMicroServiceApplication.class, "top/hunfan/mail/Func", "func", "(Ljava/lang/Object;)Ljava/lang/String;", 5, "java/lang/Object", "toString", "()Ljava/lang/String;", "(Ljava/lang/Object;)Ljava/lang/String;", new Object[0]);
    }
}
```

继承Serializable接口，lambda合成的类多出writeReplace方法，返回java.lang.invoke.SerializedLambda类，
该类的serializedLambda.getImplMethodName()方法可以返回方法名称

### mybatis-plus的 LambdaQueryWrapper类实现

1.  LambdaQueryWrapper抽象类AbstractLambdaWrapper有一个方法

```java
 protected String columnToString(SFunction<T, ?> column, boolean onlyColumn) {
        return getColumn(LambdaUtils.resolve(column), onlyColumn);
    }
```

```java
 private String getColumn(SerializedLambda lambda, boolean onlyColumn) throws MybatisPlusException {
        String fieldName = PropertyNamer.methodToProperty(lambda.getImplMethodName());
        Class<?> aClass = lambda.getInstantiatedType();
        if (!initColumnMap) {
            columnMap = LambdaUtils.getColumnMap(aClass);
        }
        Assert.notNull(columnMap, "can not find lambda cache for this entity [%s]", aClass.getName());
        ColumnCache columnCache = columnMap.get(LambdaUtils.formatKey(fieldName));
        Assert.notNull(columnCache, "can not find lambda cache for this property [%s] of entity [%s]",
            fieldName, aClass.getName());
        return onlyColumn ? columnCache.getColumn() : columnCache.getColumnSelect();
    }
```

原理也是借用jdk函数式接口实现Serializable接口后，序列化变成SerializedLambda类可以获取实现调用的方法。

结论：

```java
Func func=Object::toString;
```

Func函数接口有实现序列化接口，序列化后的SerializedLambda，调用getImplMethodName()方法可以得到“toString”