java基础知识-序列化/反序列化-gson基础知识

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1.Gson依赖

1.1.Gradle/Android

dependencies {
    implementation 'com.google.code.gson:gson:2.9.0'
}

1.2.maven

<dependencies>
    <!--  Gson: Java to Json conversion -->
    <dependency>
      <groupId>com.google.code.gson</groupId>
      <artifactId>gson</artifactId>
      <version>2.9.0</version>
      <scope>compile</scope>
    </dependency>
</dependencies>

1.2.Gson简单实用

1.2.1.基础类型

// Serialization
Gson gson = new Gson();
gson.toJson(1);            // ==> 1
gson.toJson("abcd");       // ==> "abcd"
gson.toJson(new Long(10)); // ==> 10
int[] values = { 1 };
gson.toJson(values);       // ==> [1]

// Deserialization
int one = gson.fromJson("1", int.class);
Integer one = gson.fromJson("1", Integer.class);
Long one = gson.fromJson("1", Long.class);
Boolean false = gson.fromJson("false", Boolean.class);
String str = gson.fromJson("\"abc\"", String.class);
String[] anotherStr = gson.fromJson("[\"abc\"]", String[].class);

1.2.2.对象

class BagOfPrimitives {
  private int value1 = 1;
  private String value2 = "abc";
  private transient int value3 = 3;
  BagOfPrimitives() {
    // no-args constructor
  }
}

// Serialization
BagOfPrimitives obj = new BagOfPrimitives();
Gson gson = new Gson();
String json = gson.toJson(obj);  

// ==> json is {"value1":1,"value2":"abc"}

// Deserialization
BagOfPrimitives obj2 = gson.fromJson(json, BagOfPrimitives.class);
// ==> obj2 is just like obj

Notes 如果对象内存在循环引用，序列化时将导致死循环。

例如：

@Data
public class RecursionObject {

    private String name;

    private RecursionReferObject refer;
}


@Data
public class RecursionReferObject {

    private String name;

    private RecursionObject refer;
}



public class GsonRecursionTest {


    public static void main(String[] args) {
        RecursionObject parent = new RecursionObject();
        parent.setName("1");

        RecursionReferObject son = new RecursionReferObject();
        son.setName("2");

        parent.setRefer(son);
        son.setRefer(parent);
        
        Gson gson = new GsonBuilder().create();
        String json=gson.toJson(parent);
        System.out.println(json);
        RecursionObject recursionObject=gson.fromJson(json,RecursionObject.class);
        System.out.println(recursionObject);
    }
}


/***
Exception in thread "main" java.lang.StackOverflowError
	at com.google.gson.stream.JsonWriter.string(JsonWriter.java:566)
	at com.google.gson.stream.JsonWriter.writeDeferredName(JsonWriter.java:402)
	at com.google.gson.stream.JsonWriter.value(JsonWriter.java:417)
	at com.google.gson.internal.bind.TypeAdapters$16.write(TypeAdapters.java:406)
	at com.google.gson.internal.bind.TypeAdapters$16.write(TypeAdapters.java:390)
/**

说明：

• 推荐对象字段使用基础类型
• 不需要添加给字段添加注解来表示该字段需要序列化，因为当前类(所有父类)中的所有字段默认都会被序列化
• 如果一个字段被标记为transient，默认它在序列化/反序列化时会被忽略
• null的处理
  • 当序列化时，一个null字段会被省略
  • 当反序列化时，如果一个字段找不到，则对应的对象字段会被设置为以下默认值：对象类型为null，数值类型为0，boolean类型为false
• 被synthetic 标记的字段，也会在序列化/反序列化过程中被忽略
• 内部类、匿名类、本地类所对应的外部类字段，在序列化/反序列化过程中也将会忽略(这块没太理解)

1.2.3.内部类(没看太懂)

Gson can serialize static nested classes quite easily.

Gson can also deserialize static nested classes. However, Gson can not automatically deserialize the pure inner classes since their no-args constructor also need a reference to the containing Object which is not available at the time of deserialization. You can address this problem by either making the inner class static or by providing a custom InstanceCreator for it. Here is an example:

public class A { 
  public String a; 

  class B { 

    public String b; 

    public B() {
      // No args constructor for B
    }
  } 
}

NOTE: The above class B can not (by default) be serialized with Gson.

Gson can not deserialize {"b":"abc"} into an instance of B since the class B is an inner class. If it was defined as static class B then Gson would have been able to deserialize the string. Another solution is to write a custom instance creator for B.

public class InstanceCreatorForB implements InstanceCreator<A.B> {
  private final A a;
  public InstanceCreatorForB(A a)  {
    this.a = a;
  }
  public A.B createInstance(Type type) {
    return a.new B();
  }
}

The above is possible, but not recommended.

1.2.4.Array

Gson gson = new Gson();
int[] ints = {1, 2, 3, 4, 5};
String[] strings = {"abc", "def", "ghi"};

// Serialization
gson.toJson(ints);     // ==> [1,2,3,4,5]
gson.toJson(strings);  // ==> ["abc", "def", "ghi"]

// Deserialization
int[] ints2 = gson.fromJson("[1,2,3,4,5]", int[].class); 
// ==> ints2 will be same as ints

支持多维。

1.2.5.集合

Gson gson = new Gson();
Collection<Integer> ints = Lists.immutableList(1,2,3,4,5);

// Serialization
String json = gson.toJson(ints);  // ==> json is [1,2,3,4,5]

// Deserialization
Type collectionType = new TypeToken<Collection<Integer>>(){}.getType();
Collection<Integer> ints2 = gson.fromJson(json, collectionType);
// ==> ints2 is same as ints

限制：gson可以序列化任意对象的集合，但是反序列化时需要指定集合元素的类型。

1.2.泛型

1.2.1.TypeToken的使用

1.2.1.1.对象类型的泛型

class Foo<T> {
  T value;
}
Gson gson = new Gson();
Foo<Bar> foo = new Foo<Bar>();
gson.toJson(foo); // May not serialize foo.value correctly

gson.fromJson(json, foo.getClass()); // Fails to deserialize foo.value as Bar

Type fooType = new TypeToken<Foo<Bar>>() {}.getType();
gson.toJson(foo, fooType);
gson.fromJson(json, fooType);

通过TypeToken来定义泛型类型。

1.2.1.2.集合类型的泛型

@Data
public class Bar {

    private String name;
}

public class GsonListTest {

    public static void main(String[] args) {
        Gson gson = new GsonBuilder().create();

        List<Bar> bars = new ArrayList<>();

        Bar bar = new Bar();
        bar.setName("bar 1");

        bars.add(bar);
        String json = gson.toJson(bars);
        System.out.println(json);
        Type type = new TypeToken<List<Bar>>(){}.getType();
        List<Bar> dbars = gson.fromJson(json,type);
        System.out.println(dbars);

    }
}

/**
[{"name":"bar 1"}]
[Bar(name=bar 1)]
***/

1.2.2.自定义ParameterizedType

在实际项目中，如果使用大量使用TypeToken，定义起来会比较麻烦，查看TypeToken的底层源码，发现它也是通过ParameterizedType来实现的。(不懂ParameterizedType的可以先百度一下)

public class MyParameterizedType implements ParameterizedType {

    private Type[] args;

    private Class rawType;

    public MyParameterizedType( Class rawType,Type[] args) {
        this.args = args;
        this.rawType = rawType;
    }

    @Override
    public Type[] getActualTypeArguments() {
        return args;
    }

    @Override
    public Type getRawType() {
        return rawType;
    }

    @Override
    public Type getOwnerType() {
        return null;
    }
}


//测试复杂泛型类型

public class ParameterizedTypeTest {

    public static void main(String[] args) {
        Gson gson = new GsonBuilder().create();
        Result<List<Bar>> result = new Result<>();
        List<Bar> bars = new ArrayList<>();
        Bar bar = new Bar();
        bar.setName("bar 1");
        bars.add(bar);
        result.setData(bars);
      
        Type inner = new MyParameterizedType(List.class, new Class[]{Bar.class});
        MyParameterizedType type = new MyParameterizedType(Result.class,new Type[]{inner});
      
        String json = gson.toJson(result);
        System.out.println(json);
        Result<List<Bar>> result1=gson.fromJson(json,type);
        System.out.println(result1);

    }
}

1.3.null值处理

Gson gson = new GsonBuilder().serializeNulls().create();



public class Foo {
  private final String s;
  private final int i;

  public Foo() {
    this(null, 5);
  }

  public Foo(String s, int i) {
    this.s = s;
    this.i = i;
  }
}

Gson gson = new GsonBuilder().serializeNulls().create();
Foo foo = new Foo();
String json = gson.toJson(foo);
System.out.println(json);

json = gson.toJson(null);
System.out.println(json);

{"s":null,"i":5}
null

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