Gson反序列化原理
原理简述
gson反序列化主要分为两个过程：
对象的创建
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);
}
};
}
// 获取无参构造函数
ObjectConstructor<T> defaultConstructor = newDefaultConstructor(rawType);
if (defaultConstructor != null) {
return defaultConstructor;
}
// 获取List<T>,Map<T>等构造函数，对于List,Map的情况
ObjectConstructor<T> 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
private <T> ObjectConstructor<T> newDefaultConstructor(Class<? super T> rawType) {
try {
// 获取无参构造函数
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);
}
}
};
}
结论
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
class User(val id: Int, val name: String = "sss") {
init {
println("init")
}
}
反编译的Java代码
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
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
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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