blocks|key|274264|text|您需要获取类，找到接受下限为T的单个参数的构造函数(在本例中为Object)，强制该构造函数可访问(使用setAccessible方法)，最后使用所需的参数调用它。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|274265|entityMap^0|1G|D|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

you would need to get the class, find the constructor which takes a single argument with the lower bound of T (in this case Object), force the constructor to be accessible (using the <code>setAccessible</code> method), and finally invoke it with the desired argument.

blocks|key|272732|text|确保在获取构造函数时使用getDeclaredConstructors，并将其可访问性设置为true，因为它是私有的。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|272733|像这样的东西应该行得通。|272734|Constructor<Foo>+constructor=+(Constructor<Foo>)+Foo.class.getDeclaredConstructors()[0];
constructor.setAccessible(true);+
Foo+obj+=+constructor.newInstance("foo");+
System.out.println(obj);|code-block|syntax|javascript|272735|更新|BOLD|272736|如果要使用getDeclaredConstructor，请将Object.class作为参数传递，该参数将转换为泛型T。|272737|Class+fooClazz+=+Class.forName("path.to.package.Foo");
Constructor<Foo>+constructor+=+fooClazz.getDeclaredConstructor(Object.class);
constructor.setAccessible(true);+
Foo+obj+=+constructor.newInstance("foo");+
System.out.println(obj);|272738|entityMap^0|C|N|0|0|0|0|2|0|0|0^^$0|@$1|2|3|4|5|6|7|V|8|@$9|W|A|X|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Y|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|Z|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|10|8|@$9|11|A|12|B|O]]|D|@]|E|$]]|$1|P|3|Q|5|6|7|13|8|@]|D|@]|E|$]]|$1|R|3|S|5|J|7|14|8|@]|D|@]|E|$K|L]]|$1|T|3|-4|5|6|7|15|8|@]|D|@]|E|$]]]|U|$]]

Make sure you use <code>getDeclaredConstructors</code> when getting the constructor and set its accessibility to true since its private.

Something like this should work.

<pre><code>Constructor&lt;Foo&gt; constructor= (Constructor&lt;Foo&gt;) Foo.class.getDeclaredConstructors()[0];
constructor.setAccessible(true); 
Foo obj = constructor.newInstance("foo"); 
System.out.println(obj);
</code></pre>

Update

If you want to make use of getDeclaredConstructor, pass Object.class as an argument which translates to a generic T.

<pre><code>Class fooClazz = Class.forName("path.to.package.Foo");
Constructor&lt;Foo&gt; constructor = fooClazz.getDeclaredConstructor(Object.class);
constructor.setAccessible(true); 
Foo obj = constructor.newInstance("foo"); 
System.out.println(obj);
</code></pre>

blocks|key|274320|text|有一个用于JUnit的库(dp4j)，可以自动插入用于访问私有方法的代码。这可能是有用的。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|274321|entityMap|0|LINK|mutability|MUTABLE|url|http://junit.org/node/591^0|D|4|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

There is a library for JUnit (<a href="http://junit.org/node/591" rel="nofollow">dp4j</a>) that automatically inserts code for accessing private methods. That may be of use.

blocks|key|270689|text|正如@ArtB所说，如果您知道要在编译时中使用的构造函数，则可以使用dp4j.com、。在项目主页上有一个访问Singleton构造函数的例子。|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|270690|用@Reflect注解注入反射的方法，而不是JUnit的@Test：|270691|public+class+Example+{
++++@com.dp4j.Reflect
++++public+static+void+main(final+String[]+args){
++++++++Foo<String>+foo+=+new+Foo("hello");
++++++++System.out.println(foo);
++++}+++
}|code-block|syntax|javascript|270692|要查看反射生成的代码，请使用this+answer中的-Averbose=true参数。|270693|entityMap|0|LINK|mutability|MUTABLE|url|http://dp4j.com/|1|https://stackoverflow.com/questions/5454249/java-reflection-getmethodstring-method-object-class-not-working/5459362#5459362^0|17|T|Y|8|0|0|0|Y|0|0|0|18|E|B|1|0^^$0|@$1|2|3|4|5|6|7|Y|8|@$9|Z|A|10|B|C]]|D|@$9|11|A|12|1|13]]|E|$]]|$1|F|3|G|5|6|7|14|8|@$9|15|A|16|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|17|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|18|8|@$9|19|A|1A|B|C]]|D|@$9|1B|A|1C|1|1D]]|E|$]]|$1|O|3|-4|5|6|7|1E|8|@]|D|@]|E|$]]]|P|$Q|$5|R|S|T|E|$U|V]]|W|$5|R|S|T|E|$U|X]]]]

As @ArtB said you could use <a href="http://dp4j.com" rel="nofollow noreferrer">dp4j.com</a>, if you know the constructor you want to use at compile-time. On the project homepage there's an example of just that, accessing a Singleton constructor.

Instead of JUnit's @Test annotate the method in which to inject the Reflection with @Reflect:

<pre><code>public class Example {
 @com.dp4j.Reflect
 public static void main(final String[] args){
 Foo&lt;String&gt; foo = new Foo("hello");
 System.out.println(foo);
 } 
}
</code></pre>

To see the reflection generated code use -Averbose=true argument as in <a href="https://stackoverflow.com/questions/5454249/java-reflection-getmethodstring-method-object-class-not-working/5459362#5459362">this answer</a>.

blocks|key|270704|text|好吧，如果私有构造函数没有接受任何参数，那么我们在创建新实例时会遇到问题，在这种情况下，在setAccessible为true之后，我们不能创建对象。即使是construct.newInstance(null);也不会为无参数构造函数创建对象。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|270705|我们可以使用反射创建以下代码的对象吗：|270706|public+class+Singleton+{

++++private+static+Singleton+instance+=+new+Singleton();

++++/*+private+constructor+*/
++++private+Singleton()+{}

++++public+static+Singleton+getDefaultInstance()+{
++++++++return+instance;
++++}
}|code-block|syntax|javascript|270707|是的，我们可以创建上面类的对象。|270708|//+reflection+concept+to+get+constructor+of+a+Singleton+class.++
Constructor<Singleton>+constructor+=+Singleton.class.getDeclaredConstructor();

//+change+the+accessibility+of+constructor+for+outside+a+class+object+creation.++
constructor.setAccessible(true);

//+creates+object+of+a+class+as+constructor+is+accessible+now.++
Singleton+secondOb+=+constructor.newInstance();

//+close+the+accessibility+of+a+constructor.
constructor.setAccessible(false);|270709|你可以参考我的博客：http://sanjaymadnani.wordpress.com/2014/04/14/singleton-design-pattern-in-java/的示例2：“急切初始化”和“反射单例违规”。|270710|entityMap|0|LINK|mutability|MUTABLE|url|http://sanjaymadnani.wordpress.com/2014/04/14/singleton-design-pattern-in-java/^0|26|S|0|0|0|0|0|A|27|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@$9|11|A|12|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|13|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|14|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|15|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|16|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|17|8|@]|D|@$9|18|A|19|1|1A]]|E|$]]|$1|S|3|-4|5|6|7|1B|8|@]|D|@]|E|$]]]|T|$U|$5|V|W|X|E|$Y|Z]]]]

Well in case if private constructor does not take any argument then we fetch problem while creating new instance, in this case after setAccessible true we can't create object.
Even <code>construct.newInstance(null);</code> won't create object for no argument constructor.

can we create object of below code using reflection:

<pre><code>public class Singleton {

 private static Singleton instance = new Singleton();

 /* private constructor */
 private Singleton() {}

 public static Singleton getDefaultInstance() {
 return instance;
 }
}
</code></pre>

Yes we can create the object of above class. 

<pre><code>// reflection concept to get constructor of a Singleton class. 
Constructor&lt;Singleton&gt; constructor = Singleton.class.getDeclaredConstructor();

// change the accessibility of constructor for outside a class object creation. 
constructor.setAccessible(true);

// creates object of a class as constructor is accessible now. 
Singleton secondOb = constructor.newInstance();

// close the accessibility of a constructor.
constructor.setAccessible(false);
</code></pre>

You can Refer: Example 2: "Eager Initialization" and "Singleton Violation by reflection" of my blog: <a href="http://sanjaymadnani.wordpress.com/2014/04/14/singleton-design-pattern-in-java/" rel="noreferrer">http://sanjaymadnani.wordpress.com/2014/04/14/singleton-design-pattern-in-java/</a>

blocks|key|2974858|text|如果Junit+test+Class+(在Test文件夹中)与实际的Class具有相同的包名，那么从Junit+Test+case中，我们可以调用所有私有方法进行测试，而不需要像dp4j这样的任何附加库。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2974859|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

If Junit Test Class (in Test Folder) has the same package name as of Actual Class, then from Junit Test case, we can call all private methods to test, without any additional library like dp4j.

blocks|key|274377|text|起初，我是使用反射来获取NoSuchMethodException的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|274378|使用以下命令：|274379|Constructor<TestClass>+constructor=+(Constructor<TestClass>)+TestClass.class.getDeclaredConstructors()[0];|code-block|syntax|javascript|274380|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

At first I was getting NoSuchMethodException using reflection.

Use this:

<pre><code>Constructor&lt;TestClass&gt; constructor= (Constructor&lt;TestClass&gt;) TestClass.class.getDeclaredConstructors()[0];
</code></pre>

This is a followup to <a href="https://stackoverflow.com/questions/2599440/accessing-the-private-constructor">this question about java private constructors</a>.

Suppose I have the following class:

<pre><code>class Foo&lt;T&gt;
{
 private T arg;
 private Foo(T t) {
 // private!
 this.arg = t;
 } 

 @Override
 public String toString() {
 return "My argument is: " + arg;
 } 
}
</code></pre>

How would I construct a <code>new Foo("hello")</code> using reflection?

ANSWER

Based on <a href="https://stackoverflow.com/questions/5629706/java-accessing-private-constructor-with-type-parameters/5629817#5629817">jtahlborn's answer</a>, the following works:

<pre><code>public class Example {
 public static void main(final String[] args) throws Exception {
 Constructor&lt;Foo&gt; constructor;
 constructor = Foo.class.getDeclaredConstructor(Object.class);
 constructor.setAccessible(true);
 Foo&lt;String&gt; foo = constructor.newInstance("arg1");
 System.out.println(foo);
 } 
}
</code></pre>

Java: accessing private constructor with type parameters

这是的后续文章。假设我有以下类：class Foo<T>{ private T arg; private Foo(T t) { // private! this.arg = t; } @Override public String toString() {...

问Java:使用类型参数访问私有构造函数
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