kotlin gson反序列化默認值失效

Gson反序列化原理

原理簡述

gson反序列化主要分爲兩個過程：

1. 根據TypeToken建立出對象
2. 根據json字符串解析數據，對對象屬性賦值

對象的建立

ConstructorConstructor.get

• 先嚐試獲取無參構造函數
• 失敗則嘗試List、Map等狀況的構造函數
• 最後使用Unsafe.newInstance兜底（此兜底不會調用構造函數，致使全部對象初始化代碼不會調用）

public <T> ObjectConstructor<T> get(TypeToken<T> typeToken) {
    final Type type = typeToken.getType();
    final Class<? super T> rawType = typeToken.getRawType();

    // first try an instance creator

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

    // Next try raw type match for instance creators
    @SuppressWarnings("unchecked") // types must agree
    final InstanceCreator<T> rawTypeCreator =
        (InstanceCreator<T>) instanceCreators.get(rawType);
    if (rawTypeCreator != null) {
      return new ObjectConstructor<T>() {
        @Override public T construct() {
          return rawTypeCreator.createInstance(type);
        }
      };
    }
    // 獲取無參構造函數
    ObjectConstructor<T> defaultConstructor = newDefaultConstructor(rawType);
    if (defaultConstructor != null) {
      return defaultConstructor;
    }

    // 獲取List<T>,Map<T>等構造函數，對於List,Map的狀況
    ObjectConstructor<T> defaultImplementation = newDefaultImplementationConstructor(type, rawType);
    if (defaultImplementation != null) {
      return defaultImplementation;
    }

    // unSafe構造出對象，不調用任何的構造函數
    // finally try unsafe
    return newUnsafeAllocator(type, rawType);
  }
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ConstructorConstructor.newDefaultConstructor

• 調用Class.getDeclaredConstructor獲取無參構造函數

private <T> ObjectConstructor<T> newDefaultConstructor(Class<? super T> rawType) {
    try {
        // 獲取無參構造函數
      final Constructor<? super T> constructor = rawType.getDeclaredConstructor();
      if (!constructor.isAccessible()) {
        accessor.makeAccessible(constructor);
      }
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ConstructorConstructor.newUnsafeAllocator

• 調用UnSafe.newInstance建立出對象
• 不會調用構造函數，所以全部的初始化的代碼都不會被調用

private <T> ObjectConstructor<T> newUnsafeAllocator(
      final Type type, final Class<? super T> rawType) {
    return new ObjectConstructor<T>() {
      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);
        }
      }
    };
  }
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結論

• Gson反序列要工做正常，使結果符合預期的話，要求類必須有一個無參構造函數

kotlin構造函數默認參數和無參構造函數的關係

參數裏面存在沒有默認值的狀況

kotlin代碼

• id沒有默認值

class User(val id: Int, val name: String = "sss") {
    init {
        println("init")
    }
}
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反編譯的Java代碼

• 包含兩個構造函數，一個是咱們聲明的全參數構造函數，另外一個是kotlin生成的輔助構造函數
• 不包含無參構造函數

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);
   }
}

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gson反序列化輸出

代碼：

@Test
    fun testJson() {
        val user = Gson().fromJson("{}", User::class.java)
        print(user.name)
    }
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輸出：不符合預期（咱們聲明的非空的name實際結果是null）

null
Process finished with exit code 0
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參數都包含默認參數的狀況

kotlin代碼

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

• 除了上面的兩個構造函數，多了一個無參構造函數（從邏輯上講，這個也符合預期）

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);
   }

    // 無參構造函數
   public User() {
      this(0, (String)null, 3, (DefaultConstructorMarker)null);
   }
}

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gson反序列化輸出

代碼：

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

init
sss
Process finished with exit code 0
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Best Practice

Practice1

• 屬性聲明在構造函數，全部參數都帶默認值
• 不肯定的參數聲明爲可空

class User(val id: Int=1 , val name: String = "sss") {
    init {
        println("init")
    }
}
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Practice2

• 迴歸到Java的寫法便可

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

    init {
        println("init")
    }
}
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