Gson反序列化原理
原理簡述
gson反序列化主要分為兩個過程：
對象的創建
ConstructorConstructor.get
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，對于List,Map的情況 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); }ConstructorConstructor.newDefaultConstructor
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); }ConstructorConstructor.newUnsafeAllocator
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); } } }; }結論
kotlin構造函數默認參數和無參構造函數的關系
參數里面存在沒有默認值的情況
kotlin代碼
class User(val id: Int, val name: String = "sss") { init { println("init") }}反編譯的Java代碼
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反序列化輸出
代碼：
@Test fun testJson() { val user = Gson().fromJson("{}", User::class.java) print(user.name) }輸出：不符合預期（我們聲明的非空的name實際結果是null）
null
Process finished with exit code 0
參數都包含默認參數的情況
kotlin代碼
class User(val id: Int=1, val name: String = "sss") { init { println("init") }}反編譯Java代碼
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); }}gson反序列化輸出
代碼：
@Test fun testJson() { val user = Gson().fromJson("{}", User::class.java) print(user.name) }輸出：符合預期
init
sss
Process finished with exit code 0
Best Practice
Practice1
class User(val id: Int=1 , val name: String = "sss") { init { println("init") }}Practice2
回歸到Java的寫法即可
class User { val id: Int = 1 val name: String = "sss" init { println("init") }}總結
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