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[杂七杂八] Java中有5种创建对象的方式

楼主

发表于 2017-06-09 18:17:09 | 查看: 679 | 回复: 0

Java中有5种创建对象的方式，下面给出它们的例子还有它们的字节码
图片描述
如果你运行了末尾的的程序，你会发现方法1,2,3用构造函数创建对象，方法4,5没有调用构造函数。

1.使用new关键字

这是最常见也是最简单的创建对象的方式了。通过这种方式，我们可以调用任意的构造函数(无参的和带参数的)。

< class="ttyprint" style="overflow:auto; font-family:"Source Code Pro",monospace; font-size:13px; padding:9.5px; margin-top:0px; margin-bottom:10px; line-height:1.45; color:rgb(51,51,51); word-break:break-all; word-wrap:break-word; background-color:rgb(248,249,250); text-align:justify">Employee emp1 = new Employee();< class="ttyprint" style="overflow:auto; font-family:"Source Code Pro",monospace; font-size:13px; padding:9.5px; margin-top:0px; margin-bottom:10px; line-height:1.45; color:rgb(51,51,51); word-break:break-all; word-wrap:break-word; background-color:rgb(248,249,250); text-align:justify">

0: new           #19          // class org/programming/mitra/exercises/Employee3: dup4: invokespecial #21          // Method org/programming/mitra/exercises/Employee."":()V

2.使用Class类的newInstance方法

我们也可以使用Class类的newInstance方法创建对象。这个newInstance方法调用无参的构造函数创建对象。

我们可以通过下面方式调用newInstance方法创建对象:

或者

< class="ttyprint" style="overflow:auto; font-family:"Source Code Pro",monospace; font-size:13px; padding:9.5px; margin-top:0px; margin-bottom:10px; line-height:1.45; color:rgb(51,51,51); word-break:break-all; word-wrap:break-word; background-color:rgb(248,249,250); text-align:justify">Employee emp2 = Employee.class.newInstance();< class="ttyprint" style="overflow:auto; font-family:"Source Code Pro",monospace; font-size:13px; padding:9.5px; margin-top:0px; margin-bottom:10px; line-height:1.45; color:rgb(51,51,51); word-break:break-all; word-wrap:break-word; background-color:rgb(248,249,250); text-align:justify">51: invokevirtual #70 // Method java/lang/Class.newInstance:()Ljava/lang/Object;

3.使用Constructor类的newInstance方法

和Class类的newInstance方法很像， java.lang.reflect.Constructor类里也有一个newInstance方法可以创建对象。我们可以通过这个newInstance方法调用有参数的和私有的构造函数。

< class="ttyprint" style="overflow:auto; font-family:"Source Code Pro",monospace; font-size:13px; padding:9.5px; margin-top:0px; margin-bottom:10px; line-height:1.45; color:rgb(51,51,51); word-break:break-all; word-wrap:break-word; background-color:rgb(248,249,250); text-align:justify">Constructor<Employee> constructor = Employee.class.getConstructor();Employee emp3 = constructor.newInstance();< class="ttyprint" style="overflow:auto; font-family:"Source Code Pro",monospace; font-size:13px; padding:9.5px; margin-top:0px; margin-bottom:10px; line-height:1.45; color:rgb(51,51,51); word-break:break-all; word-wrap:break-word; background-color:rgb(248,249,250); text-align:justify">111: invokevirtual #80 // Method java/lang/reflect/Constructor.newInstance:([Ljava/lang/Object;)Ljava/lang/Object;

这两种newInstance方法就是大家所说的反射。事实上Class的newInstance方法内部调用Constructor的newInstance方法。这也是众多框架，如Spring、Hibernate、Struts等使用后者的原因。想了解这两个newInstance方法的区别，请看这篇Creating objects through Reflection in Java with Example.

4.使用clone方法

无论何时我们调用一个对象的clone方法，jvm就会创建一个新的对象，将前面对象的内容全部拷贝进去。用clone方法创建对象并不会调用任何构造函数。

要使用clone方法，我们需要先实现Cloneable接口并实现其定义的clone方法。

< class="ttyprint" style="overflow:auto; font-family:"Source Code Pro",monospace; font-size:13px; padding:9.5px; margin-top:0px; margin-bottom:10px; line-height:1.45; color:rgb(51,51,51); word-break:break-all; word-wrap:break-word; background-color:rgb(248,249,250); text-align:justify">Employee emp4 = (Employee) emp3.clone();< class="ttyprint" style="overflow:auto; font-family:"Source Code Pro",monospace; font-size:13px; padding:9.5px; margin-top:0px; margin-bottom:10px; line-height:1.45; color:rgb(51,51,51); word-break:break-all; word-wrap:break-word; background-color:rgb(248,249,250); text-align:justify">162: invokevirtual #87 // Method org/programming/mitra/exercises/Employee.clone ()Ljava/lang/Object;

5.使用反序列化

当我们序列化和反序列化一个对象，jvm会给我们创建一个单独的对象。在反序列化时，jvm创建对象并不会调用任何构造函数。
为了反序列化一个对象，我们需要让我们的类实现Serializable接口

ObjectInputStream in = new ObjectInputStream(new FileInputStream("data.obj"));Employee emp5 = (Employee) in.readObject();

我们从上面的字节码片段可以看到，除了第1个方法，其他4个方法全都转变为invokevirtual(创建对象的直接方法)，第一个方法转变为两个调用，new和invokespecial(构造函数调用)。

例子

让我们看一看为下面这个Employee类创建对象：

class Employee implements Cloneable, Serializable {    private static final long serialVersionUID = 1L;    private String name;    public Employee() {        System.out.println("Employee Constructor Called...");    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    @Override    public int hashCode() {        final int prime = 31;        int result = 1;        result = prime * result + ((name == null) ? 0 : name.hashCode());        return result;    }    @Override    public boolean equals(Object obj) {        if (this == obj)            return true;        if (obj == null)            return false;        if (getClass() != obj.getClass())            return false;        Employee other = (Employee) obj;        if (name == null) {            if (other.name != null)                return false;        } else if (!name.equals(other.name))            return false;        return true;    }    @Override    public String toString() {        return "Employee [name=" + name + "]";    }    @Override    public Object clone() {        Object obj = null;        try {            obj = super.clone();        } catch (CloneNotSupportedException e) {            e.printStackTrace();        }        return obj;    }}

下面的Java程序中，我们将用5种方式创建Employee对象。你可以从GitHub找到这些代码。

public class ObjectCreation {    public static void main(String... args) throws Exception {        // By using new keyword        Employee emp1 = new Employee();        emp1.setName("Naresh");        System.out.println(emp1 + ", hashcode : " + emp1.hashCode());        // By using Class class's newInstance() method        Employee emp2 = (Employee) Class.forName("org.programming.mitra.exercises.Employee")                               .newInstance();        // Or we can simply do this        // Employee emp2 = Employee.class.newInstance();        emp2.setName("Rishi");        System.out.println(emp2 + ", hashcode : " + emp2.hashCode());        // By using Constructor class's newInstance() method        Constructor<Employee> constructor = Employee.class.getConstructor();        Employee emp3 = constructor.newInstance();        emp3.setName("Yogesh");        System.out.println(emp3 + ", hashcode : " + emp3.hashCode());        // By using clone() method        Employee emp4 = (Employee) emp3.clone();        emp4.setName("Atul");        System.out.println(emp4 + ", hashcode : " + emp4.hashCode());        // By using Deserialization        // Serialization        ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("data.obj"));        out.writeObject(emp4);        out.close();        //Deserialization        ObjectInputStream in = new ObjectInputStream(new FileInputStream("data.obj"));        Employee emp5 = (Employee) in.readObject();        in.close();        emp5.setName("Akash");        System.out.println(emp5 + ", hashcode : " + emp5.hashCode());    }}

程序会输出：

Employee Constructor Called...Employee [name=Naresh], hashcode : -1968815046Employee Constructor Called...Employee [name=Rishi], hashcode : 78970652Employee Constructor Called...Employee [name=Yogesh], hashcode : -1641292792Employee [name=Atul], hashcode : 2051657Employee [name=Akash], hashcode : 63313419

[杂七杂八] Java中有5种创建对象的方式

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