A breakdown of how objects are created in Java

Java developers create many objects every day, but they usually use dependency injection to manage the system. For example, Spring creates objects. However, there are many ways to create objects: using the New keyword and using the newInstance method of the Class class. , use the newInstance method of the Constructor class, use the Clone method, and use deserialization.

Use the new keyword: This is our most common and simplest way to create an object. In this way we can also call any Zan function (sum without parameters) There are ginseng). For example: Student student = new Student();

Use the newInstance method of the Class class: We can also use the newInstance method of the Class class to create an object. This newInstance method calls a parameterless constructor to create an object, such as: Student student2 = (Student)Class.forName("Root path.Student").newInstance(); Or: Student stu = Student.class.newInstance();

Use the newInstance method of the Constructor class: This method and Much like the newInstance method of the Class class, there is also a newInstance method in the java.lang.relect.Constructor class to create objects. We can call the parameterized and private constructor through this newInstance method. For example: Constructor constructor = Student.class.getInstance(); Student stu = constructor.newInstance(); These two newInstance methods are what everyone calls reflection. In fact, the newInstance method of Class internally calls the newInstance method of Constructor. . This is also the reason why many frameworks such as Spring, Hibernate, Struts, etc. use the latter.

Using Clone method: Whenever we call the clone method of an object, the JVM will create a new object and copy all the contents of the previous object into it. Creating an object using the clone method will not call any constructor. To use the clone method, we must first implement the Cloneable interface and implement its defined clone method. For example: Student stu2 = stu.clone(); This is also an application of prototype mode.

Use deserialization: When we serialize and deserialize an object, the JVM will create a separate object for us. During deserialization, the JVM creates the object and does not call any constructor. In order to deserialize an object, we need to make our class implement the Serializable interface. For example: ObjectInputStream in = new ObjectInputStream (new FileInputStream("data.obj")); Student stu3 = (Student)in.readObject();

As can be seen from the above example, in addition to using the new keyword All other methods are converted to invokevirtual (direct method of creating an object), and are converted into two calls by new, new and invokespecial (constructor call).

Also: Is there any difference between the two newInstance methods?

☆ The Class class is located in the lang package of java, and the constructor class is part of the java reflection mechanism.

　☆The newInstance of the Class class can only trigger the parameterless constructor to create an object, while the newInstance of the constructor class can trigger the constructor with parameters or any parameters to create an object.

　☆The newInstance of the Class class requires its constructor to be public or visible to the calling method, while the newInstance of the constructor class can call the private constructor in a specific environment to create an object.

　☆　The newInstance of the Class class throws the exception of the class constructor, and the newInstance of the constructor class wraps an InvocationTargetException exception.

The Class class essentially calls the parameterless newInstance method in the reflection package constructor class, captures the InvocationTargetException, and throws the exception of the constructor itself.

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