Previous article The common usage of member variables (java.lang.reflect.Field) in Class and the matters needing attention are introduced. Let's move on to the method in Class (java.lang.reflect.Method).
introduce
The method is an executable piece of code, which can be inherited, overloaded and rewritten, or forced to hide by the compiler. On the contrary, reflection code restricts method selection to a specific class, regardless of its parent class. Although we can find its parent class, this is not what reflection of method can do, so it is easy to cause problems here.
Get the Method declaration
Method declarations include method names, descriptors, parameters, return types, and exception tables. Class java.lang.reflect.Method provides a way to get this information.
1. Get the name of the method
String getName()
2. Getting the descriptor of the method
The return value of int getModifiers() is described in the previous article.
3. Return value type of return method
Class<?> getReturnType()
Type getGenericReturnType
4. Return method parameters (list)
Class<?>[] getParameterTypes()
Type[] getGenericParameterTypes()
5. Return method exception information
Class<?>[] getExceptionTypes()
Type[] getGenericExceptionTypes()
Why are there two ways to get method parameters, return value types, and exception tables? Because Generic is the type of the return declaration, even if the type of the declaration is generic, it will return the generic identifier instead of the true type. Last article This is illustrated by the generic erasure mentioned above.
There's no basis for words. Let's give an example.
public class MethodSpy {
private static final String fmt = "%24s: %s%n";
// for the morbidly curious
<E extends RuntimeException> void genericThrow() throws E {}
public static void main(String... args) {
try {
Class<?> c = Class.forName(args[0]);
Method[] allMethods = c.getDeclaredMethods();
for (Method m : allMethods) {
if (!m.getName().equals(args[1])) {
continue;
}
out.format("%s%n", m.toGenericString());
out.format(fmt, "ReturnType", m.getReturnType());
out.format(fmt, "GenericReturnType", m.getGenericReturnType());
Class<?>[] pType = m.getParameterTypes();
Type[] gpType = m.getGenericParameterTypes();
for (int i = 0; i < pType.length; i++) {
out.format(fmt,"ParameterType", pType[i]);
out.format(fmt,"GenericParameterType", gpType[i]);
}
Class<?>[] xType = m.getExceptionTypes();
Type[] gxType = m.getGenericExceptionTypes();
for (int i = 0; i < xType.length; i++) {
out.format(fmt,"ExceptionType", xType[i]);
out.format(fmt,"GenericExceptionType", gxType[i]);
}
}
} catch (ClassNotFoundException x) {
x.printStackTrace();
}
}
}The general meaning of this example is to enter a class and the name of the method to obtain method information.
Input results:
1. getConstructor method of java. lang. Class
$ java MethodSpy java.lang.Class getConstructor
public java.lang.reflect.Constructor<T> java.lang.Class.getConstructor
(java.lang.Class<?>[]) throws java.lang.NoSuchMethodException,
java.lang.SecurityException
ReturnType: class java.lang.reflect.Constructor
GenericReturnType: java.lang.reflect.Constructor<T>
ParameterType: class [Ljava.lang.Class;
GenericParameterType: java.lang.Class<?>[]
ExceptionType: class java.lang.NoSuchMethodException
GenericExceptionType: class java.lang.NoSuchMethodException
ExceptionType: class java.lang.SecurityException
GenericExceptionType: class java.lang.SecurityException- Case Method of java.lang.Class
$ java MethodSpy java.lang.Class cast
public T java.lang.Class.cast(java.lang.Object)
ReturnType: class java.lang.Object
GenericReturnType: T
ParameterType: class java.lang.Object
GenericParameterType: class java.lang.ObjectThe return value of the case method is generic and the identifier is "T", so the getGenericReturnType() method returns T, while getReturnType() returns java.lang.Object, which is the type after generic erasure.
3. Form method of java.io.PrintStream
$ java MethodSpy java.io.PrintStream format
public java.io.PrintStream java.io.PrintStream.format
(java.util.Locale,java.lang.String,java.lang.Object[])
ReturnType: class java.io.PrintStream
GenericReturnType: class java.io.PrintStream
ParameterType: class java.util.Locale
GenericParameterType: class java.util.Locale
ParameterType: class java.lang.String
GenericParameterType: class java.lang.String
ParameterType: class [Ljava.lang.Object;
GenericParameterType: class [Ljava.lang.Object;
public java.io.PrintStream java.io.PrintStream.format
(java.lang.String,java.lang.Object[])
ReturnType: class java.io.PrintStream
GenericReturnType: class java.io.PrintStream
ParameterType: class java.lang.String
GenericParameterType: class java.lang.String
ParameterType: class [Ljava.lang.Object;
GenericParameterType: class [Ljava.lang.Object;Getting information about parameters
We have a separate chapter to explain how to get information about parameters because they are special. For security and memory reasons, class bytecode files do not store the names of parameters, such as some parameter names, such as secret or password, which may disclose information about security-sensitive methods, such as long parameters, storage. Its parameter name causes memory explosion.
Of course, when we add the - parameters parameter to the execution, we can force the parameter name to be stored, which will not be stored by default.
The official demo code has a print parameter:
public class MethodParameterSpy {
private static final String fmt = "%24s: %s%n";
// for the morbidly curious
<E extends RuntimeException> void genericThrow() throws E {}
public static void printClassConstructors(Class c) {
Constructor[] allConstructors = c.getConstructors();
out.format(fmt, "Number of constructors", allConstructors.length);
for (Constructor currentConstructor : allConstructors) {
printConstructor(currentConstructor);
}
Constructor[] allDeclConst = c.getDeclaredConstructors();
out.format(fmt, "Number of declared constructors",
allDeclConst.length);
for (Constructor currentDeclConst : allDeclConst) {
printConstructor(currentDeclConst);
}
}
public static void printClassMethods(Class c) {
Method[] allMethods = c.getDeclaredMethods();
out.format(fmt, "Number of methods", allMethods.length);
for (Method m : allMethods) {
printMethod(m);
}
}
public static void printConstructor(Constructor c) {
out.format("%s%n", c.toGenericString());
Parameter[] params = c.getParameters();
out.format(fmt, "Number of parameters", params.length);
for (int i = 0; i < params.length; i++) {
printParameter(params[i]);
}
}
public static void printMethod(Method m) {
out.format("%s%n", m.toGenericString());
out.format(fmt, "Return type", m.getReturnType());
out.format(fmt, "Generic return type", m.getGenericReturnType());
Parameter[] params = m.getParameters();
for (int i = 0; i < params.length; i++) {
printParameter(params[i]);
}
}
public static void printParameter(Parameter p) {
out.format(fmt, "Parameter class", p.getType());
out.format(fmt, "Parameter name", p.getName());
out.format(fmt, "Modifiers", p.getModifiers());
out.format(fmt, "Is implicit?", p.isImplicit());
out.format(fmt, "Is name present?", p.isNamePresent());
out.format(fmt, "Is synthetic?", p.isSynthetic());
}
public static void main(String... args) {
try {
printClassConstructors(Class.forName(args[0]));
printClassMethods(Class.forName(args[0]));
} catch (ClassNotFoundException x) {
x.printStackTrace();
}
}
}The parameter obtained is Parameter, and like Field, there are also several methods:
getType(): parameter type
getName(): The parameter name, if the compiler adds the parameter - parameters, returns the true parameter name. If not, the parameter will be in the form of Arg N, N is the number of parameters.
getModifiers(): Parameter identifier, not detailed.
isImplicit(): If it is implicitly declared, it returns true. For example, internal classes implicitly declare parent member variables and constructors.
Our code:
public class MethodParameterExamples {
public class InnerClass { }
}The code that the compiler actually generates:
public class MethodParameterExamples {
public class InnerClass {
final MethodParameterExamples parent;
InnerClass(final MethodParameterExamples this$0) {
parent = this$0;
}
}
}isNamePresent(): Whether the name can also be related to - parameters.
isSynthetic(): Was it generated by the compiler?
For example, use the above code to get the method information of the following classes:
public class ExampleMethods<T> {
public boolean simpleMethod(String stringParam, int intParam) {
System.out.println("String: " + stringParam + ", integer: " + intParam);
return true;
}
public int varArgsMethod(String... manyStrings) {
return manyStrings.length;
}
public boolean methodWithList(List<String> listParam) {
return listParam.isEmpty();
}
public <T> void genericMethod(T[] a, Collection<T> c) {
System.out.println("Length of array: " + a.length);
System.out.println("Size of collection: " + c.size());
}
}The results of compiler plus-parameters execution are as follows:
Number of constructors: 1
Constructor #1
public ExampleMethods()
Number of declared constructors: 1
Declared constructor #1
public ExampleMethods()
Number of methods: 4
Method #1
public boolean ExampleMethods.simpleMethod(java.lang.String,int)
Return type: boolean
Generic return type: boolean
Parameter class: class java.lang.String
Parameter name: stringParam
Modifiers: 0
Is implicit?: false
Is name present?: true
Is synthetic?: false
Parameter class: int
Parameter name: intParam
Modifiers: 0
Is implicit?: false
Is name present?: true
Is synthetic?: false
Method #2
public int ExampleMethods.varArgsMethod(java.lang.String...)
Return type: int
Generic return type: int
Parameter class: class [Ljava.lang.String;
Parameter name: manyStrings
Modifiers: 0
Is implicit?: false
Is name present?: true
Is synthetic?: false
Method #3
public boolean ExampleMethods.methodWithList(java.util.List<java.lang.String>)
Return type: boolean
Generic return type: boolean
Parameter class: interface java.util.List
Parameter name: listParam
Modifiers: 0
Is implicit?: false
Is name present?: true
Is synthetic?: false
Method #4
public <T> void ExampleMethods.genericMethod(T[],java.util.Collection<T>)
Return type: void
Generic return type: void
Parameter class: class [Ljava.lang.Object;
Parameter name: a
Modifiers: 0
Is implicit?: false
Is name present?: true
Is synthetic?: false
Parameter class: interface java.util.Collection
Parameter name: c
Modifiers: 0
Is implicit?: false
Is name present?: true
Is synthetic?: falseExecution results without parameters:
Number of constructors: 1
public reflect.ExampleMethods()
Number of parameters: 0
Number of declared constructors: 1
public reflect.ExampleMethods()
Number of parameters: 0
Number of methods: 4
public boolean reflect.ExampleMethods.simpleMethod(java.lang.String,int)
Return type: boolean
Generic return type: boolean
Parameter class: class java.lang.String
Parameter name: arg0
Modifiers: 0
Is implicit?: false
Is name present?: false
Is synthetic?: false
Parameter class: int
Parameter name: arg1
Modifiers: 0
Is implicit?: false
Is name present?: false
Is synthetic?: false
public int reflect.ExampleMethods.varArgsMethod(java.lang.String...)
Return type: int
Generic return type: int
Parameter class: class [Ljava.lang.String;
Parameter name: arg0
Modifiers: 0
Is implicit?: false
Is name present?: false
Is synthetic?: false
public boolean reflect.ExampleMethods.methodWithList(java.util.List<java.lang.String>)
Return type: boolean
Generic return type: boolean
Parameter class: interface java.util.List
Parameter name: arg0
Modifiers: 0
Is implicit?: false
Is name present?: false
Is synthetic?: false
public <T> void reflect.ExampleMethods.genericMethod(T[],java.util.Collection<T>)
Return type: void
Generic return type: void
Parameter class: class [Ljava.lang.Object;
Parameter name: arg0
Modifiers: 0
Is implicit?: false
Is name present?: false
Is synthetic?: false
Parameter class: interface java.util.Collection
Parameter name: arg1
Modifiers: 0
Is implicit?: false
Is name present?: false
Is synthetic?: falseGet the identifier of the method
int getModifiers get identifiers, so here's an example, without going into details.
public class MethodModifierSpy {
private static int count;
private static synchronized void inc() { count++; }
private static synchronized int cnt() { return count; }
public static void main(String... args) {
try {
Class<?> c = Class.forName(args[0]);
Method[] allMethods = c.getDeclaredMethods();
for (Method m : allMethods) {
if (!m.getName().equals(args[1])) {
continue;
}
out.format("%s%n", m.toGenericString());
out.format(" Modifiers: %s%n",
Modifier.toString(m.getModifiers()));
out.format(" [ synthetic=%-5b var_args=%-5b bridge=%-5b ]%n",
m.isSynthetic(), m.isVarArgs(), m.isBridge());
inc();
}
out.format("%d matching overload%s found%n", cnt(),
(cnt() == 1 ? "" : "s"));
} catch (ClassNotFoundException x) {
x.printStackTrace();
}
}
}Output results:
$ java MethodModifierSpy java.lang.Object wait
public final void java.lang.Object.wait() throws java.lang.InterruptedException
Modifiers: public final
[ synthetic=false var_args=false bridge=false ]
public final void java.lang.Object.wait(long,int)
throws java.lang.InterruptedException
Modifiers: public final
[ synthetic=false var_args=false bridge=false ]
public final native void java.lang.Object.wait(long)
throws java.lang.InterruptedException
Modifiers: public final native
[ synthetic=false var_args=false bridge=false ]
3 matching overloads found
$ java MethodModifierSpy java.lang.StrictMath toRadians
public static double java.lang.StrictMath.toRadians(double)
Modifiers: public static strictfp
[ synthetic=false var_args=false bridge=false ]
1 matching overload found
$ java MethodModifierSpy MethodModifierSpy inc
private synchronized void MethodModifierSpy.inc()
Modifiers: private synchronized
[ synthetic=false var_args=false bridge=false ]
1 matching overload found
$ java MethodModifierSpy java.lang.Class getConstructor
public java.lang.reflect.Constructor<T> java.lang.Class.getConstructor
(java.lang.Class<T>[]) throws java.lang.NoSuchMethodException,
java.lang.SecurityException
Modifiers: public transient
[ synthetic=false var_args=true bridge=false ]
1 matching overload found
$ java MethodModifierSpy java.lang.String compareTo
public int java.lang.String.compareTo(java.lang.String)
Modifiers: public
[ synthetic=false var_args=false bridge=false ]
public int java.lang.String.compareTo(java.lang.Object)
Modifiers: public volatile
[ synthetic=true var_args=false bridge=true ]
2 matching overloads found