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java: add SafePublication query (P2)
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‎java/ql/src/Likely Bugs/Concurrency/SafePublication.java

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public class SafePublication {
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private Object value;
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public synchronized void produce() {
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value = new Object(); // Safely published using synchronization
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}
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public synchronized Object getValue() {
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return value;
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}
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}

‎java/ql/src/Likely Bugs/Concurrency/SafePublication.qhelp

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<!DOCTYPE qhelp PUBLIC
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"-//Semmle//qhelp//EN"
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"qhelp.dtd">
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<qhelp>
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<overview>
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<p>
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In a thread-safe class, values must be published safely to avoid inconsistent or unexpected behavior caused by visibility issues between threads. If a value is not safely published, one thread may see a stale or partially constructed value written by another thread, leading to subtle concurrency bugs.
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</p>
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<p>
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In particular, values of primitive types should not be initialised to anything but their default values (which for <code>Object</code> is <code>null</code>) unless this happens in a static context.
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</p>
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<p>
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Techniques for safe publication include:
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</p>
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<ul>
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<li>Using synchronized blocks or methods to ensure that a value is fully constructed before it is published.</li>
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<li>Using volatile fields to ensure visibility of changes across threads.</li>
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<li>Using thread-safe collections or classes that provide built-in synchronization, such as are found in <code>java.util.concurrent</code>.</li>
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<li>Using the <code>final</code> keyword to ensure that a reference to an object is safely published when the object is constructed.</li>
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</ul>
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</overview>
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<recommendation>
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<p>
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Choose a safe publication technique that fits your use case. If the value only needs to be written once, say for a singleton, consider using the <code>final</code> keyword. If the value is mutable and needs to be shared across threads, consider using synchronized blocks or methods, or using a thread-safe collection from <code>java.util.concurrent</code>.
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</p>
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</recommendation>
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<example>
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<p>In the following example, the value of <code>value</code> is not safely published. The <code>produce</code> method
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creates a new object and assigns it to the field <code>value</code>. However, the field is not
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declared as <code>volatile</code>, and there are no synchronization mechanisms in place to ensure
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that the value is fully constructed before it is published.</p>
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<sample src="UnsafePublication.java" />
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<p>To fix this example, declare the field <code>value</code> as <code>volatile</code>, or use
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synchronized blocks or methods to ensure that the value is fully constructed before it is
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published. We illustrate the latter with the following example:</p>
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<sample src="SafePublication.java" />
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</example>
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<references>
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<li>
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Java Language Specification, chapter 17:
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<a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-17.html#jls-17.4">Threads and Locks</a>.
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</li>
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<li>
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Java concurrency package:
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<a href="https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/package-summary.html">java.util.concurrent</a>.
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</li>
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</references>
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</qhelp>

‎java/ql/src/Likely Bugs/Concurrency/SafePublication.ql

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/**
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* @name Safe publication
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* @kind problem
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* @problem.severity error
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* @id java/safe-publication
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*/
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import java
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import semmle.code.java.ConflictingAccess
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/**
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* Holds if `v` should be the default value for the field `f`.
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* That is, `v` is an initial (or constructor) assignment of `f`.
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*/
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predicate shouldBeDefaultValueFor(Field f, Expr v) {
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v = f.getAnAssignedValue() and
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(
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v = f.getInitializer()
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or
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v.getEnclosingCallable() = f.getDeclaringType().getAConstructor()
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)
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}
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/**
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* Gets the default value for the field `f`.
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* See https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html
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* for the default values of the primitive types.
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* The default value for non-primitive types is null.
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*/
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bindingset[result]
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Expr getDefaultValue(Field f) {
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f.getType().hasName("byte") and result.(IntegerLiteral).getIntValue() = 0
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or
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f.getType().hasName("short") and result.(IntegerLiteral).getIntValue() = 0
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or
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f.getType().hasName("int") and result.(IntegerLiteral).getIntValue() = 0
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or
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f.getType().hasName("long") and
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(
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result.(LongLiteral).getValue() = "0" or
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result.(IntegerLiteral).getValue() = "0"
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)
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or
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f.getType().hasName("float") and result.(FloatLiteral).getValue() = "0.0"
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or
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f.getType().hasName("double") and result.(DoubleLiteral).getValue() = "0.0"
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or
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f.getType().hasName("char") and result.(CharacterLiteral).getCodePointValue() = 0
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or
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f.getType().hasName("boolean") and result.(BooleanLiteral).getBooleanValue() = false
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or
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not f.getType().getName() in [
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"byte", "short", "int", "long", "float", "double", "char", "boolean"
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] and
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result instanceof NullLiteral
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}
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/**
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* Holds if all constructor or initial assignments (if any) are to the default value.
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* That is, assignments by the declaration:
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* int x = 0; OK
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* int x = 3; not OK
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* or inside a constructor:
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* public c(a) {
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* x = 0; OK
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* x = 3; not OK
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* x = a; not OK
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* }
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*/
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predicate isAssignedDefaultValue(Field f) {
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forall(Expr v | shouldBeDefaultValueFor(f, v) | v = getDefaultValue(f))
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}
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predicate isSafelyPublished(Field f) {
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f.isFinal() or // TODO: Consider non-primitive types
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f.isStatic() or
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f.isVolatile() or
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isThreadSafeType(f.getType()) or
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isThreadSafeType(f.getInitializer().getType()) or
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isAssignedDefaultValue(f)
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}
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from Field f, ClassAnnotatedAsThreadSafe c
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where
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f = c.getAField() and
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not isSafelyPublished(f)
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select f, "The class $@ is marked as thread-safe, but this field is not safely published.", c,
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c.getName()

‎java/ql/src/Likely Bugs/Concurrency/UnsafePublication.java

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@ThreadSafe
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public class UnsafePublication {
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private Object value;
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public void produce() {
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value = new Object(); // Not safely published, other threads may see the default value null
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}
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public Object getValue() {
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return value;
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}
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}

‎java/ql/test/query-tests/SafePublication/SafePublication.expected

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| SafePublication.java:5:9:5:9 | z | The class $@ is marked as thread-safe, but this field is not safely published. | SafePublication.java:2:14:2:28 | SafePublication | SafePublication |
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| SafePublication.java:6:9:6:9 | w | The class $@ is marked as thread-safe, but this field is not safely published. | SafePublication.java:2:14:2:28 | SafePublication | SafePublication |
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| SafePublication.java:7:9:7:9 | u | The class $@ is marked as thread-safe, but this field is not safely published. | SafePublication.java:2:14:2:28 | SafePublication | SafePublication |
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| SafePublication.java:11:10:11:10 | d | The class $@ is marked as thread-safe, but this field is not safely published. | SafePublication.java:2:14:2:28 | SafePublication | SafePublication |
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| SafePublication.java:12:10:12:10 | e | The class $@ is marked as thread-safe, but this field is not safely published. | SafePublication.java:2:14:2:28 | SafePublication | SafePublication |
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| SafePublication.java:14:11:14:13 | arr | The class $@ is marked as thread-safe, but this field is not safely published. | SafePublication.java:2:14:2:28 | SafePublication | SafePublication |
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| SafePublication.java:17:10:17:11 | cc | The class $@ is marked as thread-safe, but this field is not safely published. | SafePublication.java:2:14:2:28 | SafePublication | SafePublication |

‎java/ql/test/query-tests/SafePublication/SafePublication.java

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@ThreadSafe
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public class SafePublication {
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int x;
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int y = 0;
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int z = 3; //$ Alert
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int w; //$ Alert
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int u; //$ Alert
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long a;
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long b = 0;
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long c = 0L;
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long d = 3; //$ Alert
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long e = 3L; //$ Alert
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int[] arr = new int[3]; //$ Alert
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float f = 0.0f;
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double dd = 00.0d;
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char cc = 'a'; //$ Alert
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char ok = '\u0000';
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public SafePublication(int a) {
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x = 0;
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w = 3; // not ok
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u = a; // not ok
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}
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public void methodLocal() {
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int i;
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}
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}

‎java/ql/test/query-tests/SafePublication/SafePublication.qlref

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query: Likely Bugs/Concurrency/SafePublication.ql
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postprocess: utils/test/InlineExpectationsTestQuery.ql

‎java/ql/test/query-tests/SafePublication/ThreadSafe.java

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public @interface ThreadSafe {
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}

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