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kotlin gson反序列化默认值失效深入讲解

时间:2018-12-28 09:29:00 编辑:猪哥 来源:转载

Gson反序列化原理

原理简述

gson反序列化主要分为两个过程:

  • 根据TypeToken创建出对象
  • 根据json字符串解析数据,对对象属性赋值

对象的创建

ConstructorConstructor.get

  • 先尝试获取无参构造函数
  • 失败则尝试List、Map等情况的构造函数
  • 最后使用Unsafe.newInstance兜底(此兜底不会调用构造函数,导致所有对象初始化代码不会调用)
public  ObjectConstructor get(TypeToken typeToken) {
 final Type type = typeToken.getType();
 final Class rawType = typeToken.getRawType();

 // first try an instance creator

 @SuppressWarnings("unchecked") // types must agree
 final InstanceCreator typeCreator = (InstanceCreator) instanceCreators.get(type);
 if (typeCreator != null) {
  return new ObjectConstructor() {
  @Override public T construct() {
   return typeCreator.createInstance(type);
  }
  };
 }

 // Next try raw type match for instance creators
 @SuppressWarnings("unchecked") // types must agree
 final InstanceCreator rawTypeCreator =
  (InstanceCreator) instanceCreators.get(rawType);
 if (rawTypeCreator != null) {
  return new ObjectConstructor() {
  @Override public T construct() {
   return rawTypeCreator.createInstance(type);
  }
  };
 }
 // 获取无参构造函数
 ObjectConstructor defaultConstructor = newDefaultConstructor(rawType);
 if (defaultConstructor != null) {
  return defaultConstructor;
 }

 // 获取List,Map等构造函数,对于List,Map的情况
 ObjectConstructor defaultImplementation = newDefaultImplementationConstructor(type, rawType);
 if (defaultImplementation != null) {
  return defaultImplementation;
 }

 // unSafe构造出对象,不调用任何的构造函数
 // finally try unsafe
 return newUnsafeAllocator(type, rawType);
 }

ConstructorConstructor.newDefaultConstructor

  • 调用Class.getDeclaredConstructor获取无参构造函数
private  ObjectConstructor newDefaultConstructor(Class rawType) {
 try {
  // 获取无参构造函数
  final Constructor constructor = rawType.getDeclaredConstructor();
  if (!constructor.isAccessible()) {
  accessor.makeAccessible(constructor);
  }

ConstructorConstructor.newUnsafeAllocator

  • 调用UnSafe.newInstance创建出对象
  • 不会调用构造函数,因此所有的初始化的代码都不会被调用
private  ObjectConstructor newUnsafeAllocator(
  final Type type, final Class rawType) {
 return new ObjectConstructor() {
  private final UnsafeAllocator unsafeAllocator = UnsafeAllocator.create();
  @SuppressWarnings("unchecked")
  @Override public T construct() {
  try {
  // 
   Object newInstance = unsafeAllocator.newInstance(rawType);
   return (T) newInstance;
  } catch (Exception e) {
   throw new RuntimeException(("Unable to invoke no-args constructor for " + type + ". "
    + "Registering an InstanceCreator with Gson for this type may fix this problem."), e);
  }
  }
 };
 }

结论

  • Gson反序列要工作正常,使结果符合预期的话,要求类必须有一个无参构造函数

kotlin构造函数默认参数和无参构造函数的关系

参数里面存在没有默认值的情况

kotlin代码

  • id没有默认值
class User(val id: Int, val name: String = "sss") {
 init {
  println("init")
 }
}

反编译的Java代码

  • 包含两个构造函数,一个是我们声明的全参数构造函数,另一个是kotlin生成的辅助构造函数
  • 不包含无参构造函数
public final class User {
 private final int id;
 @NotNull
 private final String name;
 
 public User(int id, @NotNull String name) {
  Intrinsics.checkParameterIsNotNull(name, "name");
  super();
  this.id = id;
  this.name = name;
  String var3 = "init";
  System.out.println(var3);
 }

 // $FF: synthetic method
 public User(int var1, String var2, int var3, DefaultConstructorMarker var4) {
  if ((var3 & 2) != 0) {
   var2 = "";
  }

  this(var1, var2);
 }
}

gson反序列化输出

代码:

 @Test
 fun testJson() {
  val user = Gson().fromJson("{}", User::class.java)
  print(user.name)
 }

输出:不符合预期(我们声明的非空的name实际结果是null)

null
Process finished with exit code 0

参数都包含默认参数的情况

kotlin代码

class User(val id: Int=1, val name: String = "sss") {
 init {
  println("init")
 }
}

反编译Java代码

  • 除了上面的两个构造函数,多了一个无参构造函数(从逻辑上讲,这个也符合预期)
public final class User {
 private final int id;
 @NotNull
 private final String name;

 public User(int id, @NotNull String name) {
  Intrinsics.checkParameterIsNotNull(name, "name");
  super();
  this.id = id;
  this.name = name;
  String var3 = "init";
  System.out.println(var3);
 }

 // $FF: synthetic method
 public User(int var1, String var2, int var3, DefaultConstructorMarker var4) {
  if ((var3 & 1) != 0) {
   var1 = 1;
  }

  if ((var3 & 2) != 0) {
   var2 = "";
  }

  this(var1, var2);
 }

 // 无参构造函数
 public User() {
  this(0, (String)null, 3, (DefaultConstructorMarker)null);
 }
}

gson反序列化输出

代码:

 @Test
 fun testJson() {
  val user = Gson().fromJson("{}", User::class.java)
  print(user.name)
 }

输出:符合预期

init
sss
Process finished with exit code 0

Best Practice

Practice1

  • 属性声明在构造函数,所有参数都带默认值
  • 不确定的参数声明为可空
class User(val id: Int=1 , val name: String = "sss") {
 init {
  println("init")
 }
}

Practice2

回归到Java的写法即可

class User {
 val id: Int = 1
 val name: String = "sss"

 init {
  println("init")
 }
}

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