java基础-序列化反序列化-gson基础知识

以下内容来之官网翻译，地址

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.write(TypeAdapters.java:406)
	at com.google.gson.internal.bind.TypeAdapters.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