Gson反序列化原理
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原理简述
gson反序列化主要分为两个过程：
对象的创建
ConstructorConstructor.get
publicObjectConstructor get(TypeToken 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 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
privateObjectConstructor 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
privateObjectConstructor newUnsafeAllocator( final Type type, final Class<? super T> 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
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")
}
}总结
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