前言
Mybatis-Plus可以使用LambdaQueryWrapper构造where条件，如下所示
LambdaQueryWrapper<Example> wrapper = Wrappers.lambdaQuery();
wrapper.eq(Example::getName, "user-1");
实则是借助了lambda表达式序列化原理来获取到列名的。
序列化与反序列化
lambda表达式序列化也是要求函数式接口实现Serializable接口的，lambda表达式序列化后真实存在的类为java.lang.invoke.SerializedLambda，一个lambda表达式有一个看不见的方法，方法名为writeReplace，而这个方法的返回值就是SerializedLambda。
准备函数式接口
/**
* 继承Serializable接口, 这样SFunction的实现类也就实现了Serializable接口
*/
@FunctionalInterface
public interface SFunction<T, R> extends Function<T, R>, Serializable {
}
public class User {
private Integer id;
private String name;
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
package com.wangtao.lambda.serialize;
import org.junit.Assert;
import org.junit.Test;
import java.io.*;
import java.lang.invoke.SerializedLambda;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
/**
* @author wangtao
* Created at 2022/8/9 22:31
*/
public class LambdaSerializedTest {
/**
* 程序结果:
* class com.wangtao.lambda.serialize.LambdaSerializedTest$$Lambda$4/708049632
* 反序列化回来的结果并不是java.lang.invoke.SerializedLambda, 是因为调用了SerializedLambda
* 中的readResolve方法还原成了真实的类型, 参照SerializedLambda源码说明
*/
@Test
public void fun1() {
SFunction<User, String> sFunction = User::getName;
try (ByteArrayOutputStream baos = new ByteArrayOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(baos)) {
oos.writeObject(sFunction);
oos.flush();
// 反序列化
byte[] bytes = baos.toByteArray();
try (ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(bytes))) {
Object obj = ois.readObject();
System.out.println(obj.getClass());
}
} catch (IOException | ClassNotFoundException e) {
e.printStackTrace();
}
}
}
获取SerializedLambda
第一种方式，使用反射调用writeReplace方法
@Test
public void fun2() {
SFunction<User, String> sFunction = User::getName;
try {
System.out.println(sFunction.getClass());
Method method = sFunction.getClass().getDeclaredMethod("writeReplace");
method.setAccessible(true);
SerializedLambda serializedLambda = (SerializedLambda) method.invoke(sFunction);
Assert.assertEquals("getName", serializedLambda.getImplMethodName());
Assert.assertEquals(User.class.getName().replace(".", "/"), serializedLambda.getImplClass());
} catch (NoSuchMethodException | IllegalAccessException | InvocationTargetException e) {
e.printStackTrace();
}
}
第二种方式，使用序列化反序列化获取，这里有点小技巧，从上面看，正常反序列化回来时是获取不到SerializedLambda的，但是我们可以复制一份SerializedLambda源代码，然后去掉它的readResolve方法就不会还原成真实的类了。
com.wangtao.lambda.serialize.SerializedLambda
package com.wangtao.lambda.serialize;
import java.io.Serializable;
import java.lang.invoke.MethodHandleInfo;
import java.util.Objects;
/**
* @author wangtao
* Created at 2022/8/9 22:41
*/
public final class SerializedLambda implements Serializable {
private static final long serialVersionUID = 8025925345765570181L;
private final Class<?> capturingClass;
private final String functionalInterfaceClass;
private final String functionalInterfaceMethodName;
private final String functionalInterfaceMethodSignature;
private final String implClass;
private final String implMethodName;
private final String implMethodSignature;
private final int implMethodKind;
private final String instantiatedMethodType;
private final Object[] capturedArgs;
/**
* Create a {@code SerializedLambda} from the low-level information present
* at the lambda factory site.
*
* @param capturingClass The class in which the lambda expression appears
* @param functionalInterfaceClass Name, in slash-delimited form, of static
* type of the returned lambda object
* @param functionalInterfaceMethodName Name of the functional interface
* method for the present at the
* lambda factory site
* @param functionalInterfaceMethodSignature Signature of the functional
* interface method present at
* the lambda factory site
* @param implMethodKind Method handle kind for the implementation method
* @param implClass Name, in slash-delimited form, for the class holding
* the implementation method
* @param implMethodName Name of the implementation method
* @param implMethodSignature Signature of the implementation method
* @param instantiatedMethodType The signature of the primary functional
* interface method after type variables
* are substituted with their instantiation
* from the capture site
* @param capturedArgs The dynamic arguments to the lambda factory site,
* which represent variables captured by
* the lambda
*/
public SerializedLambda(Class<?> capturingClass,
String functionalInterfaceClass,
String functionalInterfaceMethodName,
String functionalInterfaceMethodSignature,
int implMethodKind,
String implClass,
String implMethodName,
String implMethodSignature,
String instantiatedMethodType,
Object[] capturedArgs) {
this.capturingClass = capturingClass;
this.functionalInterfaceClass = functionalInterfaceClass;
this.functionalInterfaceMethodName = functionalInterfaceMethodName;
this.functionalInterfaceMethodSignature = functionalInterfaceMethodSignature;
this.implMethodKind = implMethodKind;
this.implClass = implClass;
this.implMethodName = implMethodName;
this.implMethodSignature = implMethodSignature;
this.instantiatedMethodType = instantiatedMethodType;
this.capturedArgs = Objects.requireNonNull(capturedArgs).clone();
}
/**
* Get the name of the class that captured this lambda.
* @return the name of the class that captured this lambda
*/
public String getCapturingClass() {
return capturingClass.getName().replace('.', '/');
}
/**
* Get the name of the invoked type to which this
* lambda has been converted
* @return the name of the functional interface class to which
* this lambda has been converted
*/
public String getFunctionalInterfaceClass() {
return functionalInterfaceClass;
}
/**
* Get the name of the primary method for the functional interface
* to which this lambda has been converted.
* @return the name of the primary methods of the functional interface
*/
public String getFunctionalInterfaceMethodName() {
return functionalInterfaceMethodName;
}
/**
* Get the signature of the primary method for the functional
* interface to which this lambda has been converted.
* @return the signature of the primary method of the functional
* interface
*/
public String getFunctionalInterfaceMethodSignature() {
return functionalInterfaceMethodSignature;
}
/**
* Get the name of the class containing the implementation
* method.
* @return the name of the class containing the implementation
* method
*/
public String getImplClass() {
return implClass;
}
/**
* Get the name of the implementation method.
* @return the name of the implementation method
*/
public String getImplMethodName() {
return implMethodName;
}
/**
* Get the signature of the implementation method.
* @return the signature of the implementation method
*/
public String getImplMethodSignature() {
return implMethodSignature;
}
/**
* Get the method handle kind (see {@link MethodHandleInfo}) of
* the implementation method.
* @return the method handle kind of the implementation method
*/
public int getImplMethodKind() {
return implMethodKind;
}
/**
* Get the signature of the primary functional interface method
* after type variables are substituted with their instantiation
* from the capture site.
* @return the signature of the primary functional interface method
* after type variable processing
*/
public final String getInstantiatedMethodType() {
return instantiatedMethodType;
}
/**
* Get the count of dynamic arguments to the lambda capture site.
* @return the count of dynamic arguments to the lambda capture site
*/
public int getCapturedArgCount() {
return capturedArgs.length;
}
/**
* Get a dynamic argument to the lambda capture site.
* @param i the argument to capture
* @return a dynamic argument to the lambda capture site
*/
public Object getCapturedArg(int i) {
return capturedArgs[i];
}
@Override
public String toString() {
String implKind= MethodHandleInfo.referenceKindToString(implMethodKind);
return String.format("SerializedLambda[%s=%s, %s=%s.%s:%s, " +
"%s=%s %s.%s:%s, %s=%s, %s=%d]",
"capturingClass", capturingClass,
"functionalInterfaceMethod", functionalInterfaceClass,
functionalInterfaceMethodName,
functionalInterfaceMethodSignature,
"implementation",
implKind,
implClass, implMethodName, implMethodSignature,
"instantiatedMethodType", instantiatedMethodType,
"numCaptured", capturedArgs.length);
}
}
注:
此类与java.lang.invoke.SerializedLambda基本一模一样，唯一的区别是包名不一样，并且没有readResolve方法
@Test
public void fun3() {
SFunction<User, String> sFunction = User::getName;
try (ByteArrayOutputStream baos = new ByteArrayOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(baos)) {
oos.writeObject(sFunction);
oos.flush();
// 反序列化
byte[] bytes = baos.toByteArray();
try (ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(bytes)) {
/**
* resolveClass可以允许在反序列化时替换类型, 但是需要满足下面条件
* 1. 类名必须一致, 包名可以不一样
* 2. serialVersionUID必须和原来的类一样(这个是反序列化的必要条件, 不然会报错)
*/
@Override
protected Class<?> resolveClass(ObjectStreamClass desc) throws IOException, ClassNotFoundException {
Class<?> clazz = super.resolveClass(desc);
return clazz == SerializedLambda .class ? com.wangtao.lambda.serialize.SerializedLambda.class : clazz;
}
}) {
com.wangtao.lambda.serialize.SerializedLambda serializedLambda = (com.wangtao.lambda.serialize.SerializedLambda ) ois.readObject();
Assert.assertEquals("getName", serializedLambda.getImplMethodName());
Assert.assertEquals(User.class.getName().replace(".", "/"), serializedLambda.getImplClass());
}
} catch (IOException | ClassNotFoundException e) {
e.printStackTrace();
}
}
当拿到SerializedLambda后，我们就能获取lambda表达式的很多元数据了，比如getImplMethodName()就能拿到方法引用的方法名称了，见上面程序。
总结
回到前言的例子
LambdaQueryWrapper<Example> wrapper = Wrappers.lambdaQuery();
wrapper.eq(Example::getName, "user-1");
wrapper.eq的第一个参数就是一个lambda表达式，当传递一个方法引用时，便可以使用上述两种方式获取到SerializedLambda实例，调用getImplMethodName方法就能获取到方法名getName了，通过截取就能得到属性名(列名)name了，就可以构造where条件了。